High-voltage depletion mode MOSFET

ABSTRACT

All low-temperature processes are used to fabricate a variety of semiconductor devices in a substrate the does not include an epitaxial layer. The devices include a non-isolated lateral DMOS, a non-isolated extended drain or drifted MOS device, a lateral trench DMOS, an isolated lateral DMOS, JFET and depletion-mode devices, and P-N diode clamps and rectifiers and junction terminations. Since the processes eliminate the need for high temperature processing and employ “as-implanted” dopant profiles, they constitute a modular architecture which allows devices to be added or omitted to the IC without the necessity of altering the processes used to produce the remaining devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/443,745,filed May 31, 2006, which is incorporated herein by reference in itsentirety.

This application is related to application Ser. No. 10/262,567, filedSep. 29, 2002, now U.S. Pat. No. 6,855,985, which is incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor chip fabrication and inparticular to methods of fabricating, integrating and electricallyisolating high-voltage and low-voltage bipolar, CMOS and DMOStransistors and passive components in a semiconductor chipmonolithically without the need for high temperature fabricationprocessing steps.

In the fabrication of semiconductor integrated circuit (IC) chips, it isfrequently necessary to electrically isolate devices that are formed onthe surface of the chip, especially when these components operate atdifferent voltages. Such complete electrical isolation is necessary tointegrate certain types of transistors including bipolar junctiontransistors and various metal-oxide-semiconductor (MOS) transistorsincluding power DMOS transistors. Complete isolation is also needed toallow CMOS control circuitry to float to potentials well above thesubstrate potential during operation.

Complete isolation is especially important in the fabrication of analog,power, and mixed signal integrated circuits. In many circuits andapplications it may be necessary or desirable to integrate both isolatedand non-isolated high-voltage devices on the same chip as other isolatedcomponents, with the caveat that high-voltage device fabrication shouldnot degrade the isolation's electrical properties, and that theisolation's fabrication steps should not adversely alter high-voltagedevice characteristics. There are various ways of doing this.

Conventional CMOS fabricated in P-type substrate material does notfacilitate complete isolation of its devices since every P-type wellforming the body (back-gate) of NMOS transistors is shorted to thesubstrate potential, typically the most negative on-chip potential.Epitaxial junction-isolation or epi-JI employs an N-type epitaxial layergrown atop a P-type silicon substrate and separated into electricallyisolated tubs by a deep P-type isolation diffusion—one requiring hightemperature processes to implement. High temperature processing causes aredistribution of dopant atoms in the substrate and epitaxial layers,causing unwanted tradeoffs and compromises in the manufacturing ofdissimilar devices fabricated using one common process. Moreover, thehigh-temperature diffusions and epitaxy employed in epi-JI processes aregenerally incompatible with the large wafer diameters and advancedlow-temperature processing equipment common in submicron CMOS fabs.

The Benefit of an Isolated Source-Body Short

In high voltage or power devices, there is a distinct performance andsurvivability advantage to MOS transistors integrating a source-bodyshort over those without a source-body short. Compared to conventionallogic and small-signal devices, a power or high voltage device with anintegral source-body short has distinct advantage over devices withseparate and physically remote source and body contacts.

The need for a source-body short in many power devices is a consequenceof their application and power circuit requirements. One way to quicklyaccess the electrical requirements of a power device in a givenapplication is to consider its topological relationship to the load andto its source of power. We herein refer to this relationship as a“switch-load topology”.

In FIGS. 1A and 1B, a power MOSFET connected to ground or a negativepotential is connected in series with a load connected to a positivepotential or supply V_(cc). Since the MOSFET “switch” is connected toground, we herein topologically refer to it as a low-side switch or LSS,even if it is used as a current source. In FIG. 1A, using a conventionalnon-isolated CMOS process, circuit 1 includes load 3, an LSS comprisingan NMOS 2, and a current sense resistor 4. In such a process, bodycontact of MOSFET 2 is necessarily shorted to the substrate, i.e. it isgrounded.

To measure the voltage across the sense resistor, current sensingrequires the source of NMOS 2 should not be shorted to the body andsubstrate, i.e. V_(B)≠V_(S). The voltage differential between source andbody causes a number of problems. Specifically, any voltage developedacross sense resistor 4 increases the source-to-body potential which inturn increases the MOSFET's threshold voltage (due to a phenomenon knownas the “body effect”). A high threshold in turn increases on-resistancewhile lowering saturation current, adversely impacting switchperformance. Another undesired effect of disconnecting the source andbody is any avalanche or displacement current in drain-to-body diode 5does not pass through the sense resistor and is therefore not detected.Finally, without a low resistance body contact, snapback breakdown canoccur easily.

Using LSS devices with an integral source-body short such as NMOS 12 incircuit 10 of FIG. 1B, drain-to-body diode 15 is anti-parallel to theMOSFET's drain and source terminals (i.e. reverse biased but inparallel), so that any current flowing through load 13 is detected insense resistor 14 regardless of whether this current flows through thechannel of NMOS 12 or through reverse biased diode 15. Because V_(SB)=0regardless of the source potential, no body effect is manifest, and thetransistor's conduction characteristics do not change substantially withcurrent.

The source-body short also improves avalanche-ruggedness by reducing therisk of snapback effects (discussed below), particularly if thesource-body short can be distributed uniformly across a large areadevice rather than shorted together in a single location. Integration ofa source body short into a large area NMOS, while common in discretepower devices, requires isolation of the P-type body from the P-typesubstrate in integrated form, something conventional CMOS cannot offer.Processes offering such isolation are complex to manufacture, oftenrequiring high temperature fabrication steps.

In FIGS. 1C and 1D, a power MOSFET connected to a positive potential orsupply V_(cc) is connected in series with a load connected to ground ora negative potential. Since the MOSFET “switch” is connected to thepositive supply, we herein topologically refer to it as a high-sideswitch or HSS, even if it is used as a current source.

Using a conventional non-isolated CMOS process, circuit 20 in FIG. 1Cincludes load 23 and a HSS comprising NMOS 22. In such a process, bodycontact of MOSFET 22 is necessarily shorted to the substrate, i.e. it isgrounded. When the NMOS is on and V_(S) increases to a potentialapproaching V_(cc), a large reverse biased potential −V_(SB) developsacross diode 25. The resulting body effect causes the threshold of NMOS22 to increase substantially, making it difficult to provide adequategate drive to achieve a low on-resistance without damaging the thin gateoxide of NMOS 22.

Using devices with an integral source-body short such as NMOS 32 incircuit 30 of FIG. 1D, the current in load 33 can easily be controlledwithout the need to counteract threshold variations due to body effect.In such a topology, drain-to-body diode 35 remains anti-parallel to theMOSFET's drain and source terminals (i.e. reverse biased but inparallel), and remains reversed biased under all normal operatingconditions. Because V_(SB)=0 regardless of the source potential, no bodyeffect is manifest, and the transistor's conduction characteristics donot change substantially with current. The source-body short alsoimproves avalanche-ruggedness by reducing the risk of snapback effects(discussed below), particularly if the source-body short can bedistributed uniformly across a large area device rather than shortedtogether in a single location. Integration of a source body short into alarge area NMOS, while common in discrete power devices, requiresisolation of the P-type body from the P-type substrate in integratedform, something conventional CMOS cannot offer. Processes offering suchisolation are complex to manufacture, often requiring high temperaturefabrication steps.

In FIGS. 1E, 1F and 1G a power MOSFET is employed as a bidirectionalswitch without either source or drain permanently connected to either apositive or negative supply rail. Since the MOSFET “switch” is notconnected to any supply but instead may block current or conduct currentin either direction, we herein topologically refer to it as an ACswitch, or a “pass transistor”.

Using conventional CMOS fabrication, pass transistor 40 in FIG. 1Ecomprises NMOS 41 with a grounded body connection and reverse biasedsource-to-body and drain-to-body diodes 42 and 43, respectively. Theterms “source” and “drain” are somewhat arbitrary in pass transistor orAC switch applications since it is often impossible to determine whichterminal, the one biased at V_(S) or the one biased at V_(D), will bemore positive at any given instance. Because the voltage across diodes42 and 43 is large, the body effect can cause significant changes in thethreshold, on-resistance, and saturation current of NMOS 41, making it apoor AC switch.

An alternative approach to implementing AC switch requiring at least twoNMOS devices with a source-body short is shown in circuit 45 of FIG. 1F,where NMOS transistors 46 and 47 are connected in series with a commonsource V_(S) such that drain-to-body diodes 48 and 49 are connectedback-to-back. In its off state, the gate terminal is biased to thesource terminal V_(S) thereby preventing channel conduction. Conductionthrough the anti-parallel body diodes is also prevented since one of thetwo diodes remains reversed biased regardless of the polarity appliedacross the series connected switches.

In its on state, whenever the common gate is biased above the sourceterminal, AC switch 45 may conduct current in either direction sinceboth transistors are turned on. The resulting AC switch is able to blockbi-directionally and conduct bi-directionally. Despite the fact that thevoltage V_(S) floats at a potential between V_(D1) and V_(D2), no bodyeffect is manifest since V_(SB)=0, i.e. each transistor has an integralsource body short. Such a device can easily be integrated into anyprocess having full isolation or capable of integrating DMOS devices.Without isolation, such a device cannot be integrated monolithicallywith other components or circuitry. It should also be noted that thedevices can be connected with a common drain rather than common sourcebut still need an isolated source-body short.

A disadvantage of AC switch 45 is its high specific on-resistance, i.e.a large R_(DS)A, since the two series connected transistors exhibitadditive resistances. If the switches were somehow connected inparallel, then the same area switch would exhibit a resistance onequarter that of the back-to-back approach of switch 45.

One such switch is shown in circuit 50 of FIG. 1G combing a symmetricNMOS device 51 and a body-bias generating (BBG) circuit 52. The purposeof BBG circuit 52 is to bias the body of NMOS 51 to the most negativepotential applied across the device, to reverse bias eitherdrain-to-body diode 55 or source-to-body diode 56, depending on whetherthe V_(S) or V_(D) terminal is more positive. In that way no diodeconduction ever occurs and if the gate of the transistor is biased tothe body potential, the device is off and will block bi-directionally.Conversely, since the device is symmetric, if the gate is biased “on”the device will conduct bi-directionally. Note that the nomenclature“drain” and “source” are arbitrary and used only to identify the circuitelements.

The BBG circuit shown as an example utilizes cross coupled NMOStransistors 53 and 54 to determine and bias the body potential V_(B) onNMOS 51, but in so doing, they themselves must include a source-bodyshort isolated from the substrate. So while switch 50 does not utilize aDMOS transistor such as the preferred implementation of AC switch 45does, it still needs isolation to be integrated into an IC with othercircuitry.

Suppressing Snapback Breakdown Effects

Aside from the need to integrate NMOS devices with isolated source bodyshorts, another limitation of conventional CMOS is its inability toprevent undesirable snapback breakdown effects in MOSFET operation;particularly in NMOS transistors.

Snapback breakdown refers to a phenomena leading to negative resistancein a device where for some range of operating conditions an increase incurrent corresponds to a “decrease” in the voltage sustaining capabilityof the transistor. Negative resistance is especially problematic inpower electronic circuitry, giving rise to excess currents, oscillationsand instability, electrical noise, localized heating, thermal runawayand even device destruction.

In power electronics, methods are required to prevent negativeresistance at all costs, including using special device constructioninvolving unusual design and process methods, in intentionally degradingor limiting the maximum voltage or current imposed on a device, and byother circuit and application methods. Unless a device is overheating,negative electrical resistance is generally a consequence of eitherparasitic bipolar conduction, conductivity modulation resulting fromimpact ionization, or some combination of both.

In FIG. 2A for example, a lightly doped drain NMOS 60 comprising P-typesubstrate 61, P+ substrate contact 62, N+ drain 64, N− drift region 65,MOS insulated gate 69, and N+ source 63 is biased in its on state withsome positive voltage ˜V_(CC) applied to its drain. Overlaidschematically on the device, is drain diode 59 representing thedrain-to-substrate diode current either arising from avalanche, fromimpact ionization or from junction leakage. The majority-carriersubstrate current, or “holes” flowing in P-type substrate 61 exhibits aresistive voltage drop, schematically represented by series-connectedR_(DB) and R_(SB) substrate resistances 67 and 68, respectively. Becauseof substrate resistance, the resulting voltage V_(B) in the bulksubstrate located beneath source 63 will rise to a voltage higher thanthe ground terminal connected to P+ contact 62. If this voltageapproaches a few tenths of a volt, N+ region 63 may start to injectelectrons, i.e. minority carriers, into substrate 61 which willnaturally be attracted by the two-dimensional electric fields in thedevice to the most positive potential, in this case N+ drain 64. Thiselectron conduction mechanism is represented by parasitic NPN bipolartransistor 66 comprising N+ collector 64, P-type substrate base 61, andN+ emitter 63. Since the voltage sustaining capability of a bipolartransistor is lower than a simple P-N junction diode (because of currentgain), the sustaining voltage of NPN 66 is lower than the NMOS itselfand the voltage will snapback to a lower value, BV_(CER)—a notationdescribing the bipolar's collector-to-emitter voltage and having aresistive, non-shorted, base contact.

Another mechanism leading to snapback illustrated in the cross sectionalview of FIG. 2B is impact ionization in the drain of the MOSFET. In thiscase the NMOS is biased to a high-voltage V_(CC) thereby reverse biasingthe drain-to-substrate junction comprising N+ drain 64 and P-typesubstrate 61. The voltage is dropped across a depletion regionillustrated by equipotential curves 71 at voltages 0V (substrate), V₁,V₂, V₃, V₄ and V₅, each curve in increasing magnitude of voltagepotential. The N− drift region under such a bias condition depletes,allowing the equipotential lines to cross the junction boundary betweenthe N− drift region and the substrate.

Ideally these equipotential lines should be spaced linearly along thedrift region with half the applied voltage being located at the centerof the drift region between gate 69 and N+ drain 64. Because of surfacecharge and other unavoidable surface effects, however, the equipotentiallines do not spread themselves uniformly, but instead “bunch up” nearthe gate edge resulting an a locally higher electric field at the end ofthe drift region. Even worse, the high electric field is physicallylocated near a region of high current density. In saturation when thedevice has a high drain potential while conducting current, the maincurrent path indicated by arrow 72, flows under the gate then away fromthe surface as it approaches the edge of depleted drift region 65. Theproduct of high current density and high electric field results inimpact ionization, i.e. local carrier creation, resulting fromcollisions of fast electrons with the atomic structure of the crystal.The collisions dislodge valence electrons from bonding the atomstogether, and convert them into more free conduction electrons which arein turn also accelerated by the locally high electric field.

The resulting impact ionization is herein represented by the concentriccontours 73 representing increased generation rates. Since impactionization creates electron-hole pairs, two undesirable effects result.First the electrons are accelerated to high energies relative to thecrystal, i.e. they become energetically “hot”, and may get swept intothe gate oxide damaging the dielectric. The second phenomenon is thegenerated hole current contributes to additional voltage drop across thesubstrate resistance R_(SB), exacerbating the NPN snapback effect.

At even higher impact ionization rates and high currents, anotherphenomenon occurs. In such cases the generated carriers start to alterthe local conductivity of the drift region by introducing sufficientadditional charge that it begins to alter local space charge neutrality.The extra electrons attract extra holes, which act like an increase indrift doping. The higher effective doping decreases depletion spreadinginto the N− layer and forces the equipotentials to bunch up even more,essentially increasing the local electric field at the edge of the driftregion and further increasing impact ionization. The result is anothercause of negative resistance since more impact ionization causes a highlocal field and contributes to even more current. Moreover, the twonegative resistance effects can occur simultaneously, interacting in acomplex and even unpredictable way. Regardless of the mechanism, theresult is a decrease in the drain voltage that the NMOS can sustain at agiven current.

Electrically the phenomenon of snapback is shown in graph 75 of draincurrent I_(D) versus drain voltage V_(DS) in FIG. 2C. Ideal devicebreakdown BVDSS shown by curve 76 may be substantially greater thansnapback voltage BV_(CER) shown by curve 77, even by a factor of two ormore in voltage. If the drain is driven into avalanche at high currentswhile sustaining voltage BV_(DSS), it may suddenly collapse back toBV_(CER), causing the current to increase and destroying the device. Ifthe NMOS is operating as a current source or switching from on to off,the onset of snapback may be exacerbated by increased substrate leakagedue to impact ionization. Curves 78, 69, 80, and 81 illustrate thedevice may not even be useful for operating at any voltage aboveBV_(CER).

One reason for the onset of snapback is that the R_(SB) substrateresistance 68 between and beneath N+ source 63 and body contact 62 istoo large, especially if the substrate is lightly doped. The othereffect is that the parasitic NPN gain is too great since there is notenough base charge in the lightly doped substrate. One obvious way toreduce the NPN transistor's adverse influence is to increase substratedoping, but unfortunately doing so also increases the electric field atthe drain leading to even more impact ionization and substrate current.

The snapback effect is sometimes represented schematically byillustrating the parasitic bipolar associated with a MOSFET. Forexample, circuit 85 in FIG. 2D illustrates NMOS 86 with parasitic NPN87, and nonlinear emitter to base shorting resistor 88. Similarly a PMOSincludes a parasitic PNP, but since PNP gain is much lower than NPNgain, and since hot hole induced impact ionization rates are much lowerthan electron ionization rates, the snapback phenomenon is less of anissue in a PMOS than in an NMOS.

Conventional DMOS Fabrication

One way to suppress snapback through additional channel doping and lowersubstrate resistance without increasing the drain electric field is byforming a DMOS field effect transistor. A DMOS, a name where the letter“D” stands for double (and originally for double diffused) isconstructed where the channel or body doping under the gate is notuniform, but concentrated or localized near the source side of the gateto avoid adversely increasing electric fields in the vicinity of thedrain region. In this way the channel concentration can be adjustedwithout affecting impact ionization or drain voltage breakdown voltages.

DMOS field effect transistors may be in isolated or non-isolatedversions. In conventional technology, the isolated requires the use ofepitaxial deposition, generally of N-type epitaxy grown atop a P-typesubstrate

As shown in FIG. 3A, N-type epitaxial layer 92 is grown atop P-typesubstrate 91 to form an isolated DMOS device 90, further comprising gatepolysilicon 98, gate oxide 99, N+ drain contact 94, N+ source 96, P+body contact 97 and a P-type “body” or PB region 93 unique to DMOStransistors. The N− drift region 95 is optional and may not be requiredif the epitaxial doping is sufficient to achieve low on-resistance. Theextra N− drift doping may be added to optimize the tradeoff betweenbreakdown and resistance but remains limited by impact ionizationeffects where the gate juxtaposes the drift region.

In an alternative form N-type epitaxial layer can be replaced by aP-type epitaxial layer or substrate, but then N− drift region 95 ismandatory for device operation. Without an N-type epitaxial layerhowever, the DMOS is not isolated and has its P-type body electricallyshorted to ground, i.e. to the substrate.

Conventional DMOS fabrication is shown in cross sections 100 and 105 inFIGS. 3B and 3C. As shown epitaxial layer 92 is covered by patternedphotoresist 101 and implanted by boron at a low energy to form shallowlayer 102. The implantation is performed at a low energy, typicallybetween 50 to 100 kev, and nearly perpendicular to the wafer's surface,e.g. only 3 degrees off axis, with limited lateral penetration undergate 98.

The implant is then driven in, i.e. diffused at high temperatures over along time, to extend the dopant laterally under gate 98 to form junction93 as shown in FIG. 3C. The diffusion, taking anywhere from 7 to 24hours, requires high temperatures over 1050° C. and typically 1100° C.or higher, a process incompatible with many modern low-temperaturefabrication facilities and large wafer diameters. The progression of thediffusion as shown in FIG. 3C and illustrated by diffusions 106 at timest₁, t₂ and t₃, occurs both laterally and vertically, where the lateralextent is roughly 80% of the vertical junction depth. In the versionshown, the body diffusion is self-aligned to the gate since it wasimplanted after the gate was formed.

If a low temperature process is required, another self alignedfabrication method to form a DMOS device is shown in FIG. 3D. In thistechnique the body implant is performed at a higher energy, typically atseveral hundred thousand electron volts, but more importantly at a steepangle, e.g. at 45°, to guarantee the body dopant penetrates laterallyunder gate 98 to a sufficient extent to fully enclose N+ source 96. Thelateral implant method is complex and undesirable for manufacturingsince the implant must be performed four times to cover all four gateorientations on a wafer. Rotating the wafer during implantation makesuniform implantation difficult.

Another DMOS fabrication method is to form a non-self aligned DMOS 120such as shown in FIGS. 3E to 3G. In FIG. 3E, a shallow boron implant 129is formed in epitaxial layer 122 masked by patterned photoresist 128.The implant is then diffused at high temperatures as shown in FIG. 3Ffor a long period of time. The P-type region diffuses both verticallyand laterally as illustrated by curves 123 representing the P-N junctionat increasing times t₁, t₂, t₃ and t₄. Finally in FIG. 3G, gateelectrode 125 with underlying gate oxide 126 is positioned over the edgeof the junction 124 such that the junction at the surface is locatedbetween gate edges 127A and 127B. Since it is not self-aligned therelative location of gate 125 and junction 124 is subject to maskmisalignment during manufacturing.

In every case described, the process of high temperature diffusion leadsto a monotonically decreasing dopant concentration profile of the DMOSbody region, with the highest concentration at the wafer's surface.Unfortunately such a profile means the surface electric field is higherthan in the bulk away from the surface, not ideal for manufacturingrobust avalanche-rugged devices.

Conventional Junction Isolation Fabrication

The high temperature diffusions involved in DMOS body fabrication arefurther complicated by the steps needed to achieve full electricalisolation of circuitry using epitaxial junction isolation.

In such conventional prior art processes as shown in FIGS. 4A through4I, a p-type substrate 131 is masked by photoresist 132 and implantedwith arsenic or antimony 133, then masked again by photoresist 134 andimplanted with boron 135 a shown in FIG. 4C. The implants are thendiffused at extremely high temperatures, sometimes as high as 1200° C.,and for as long as 24 hours to diffuse the slow moving antimony into thesubstrate and away from the surface prior to epitaxial growth. Duringsuch diffusions, oxide 138 is grown to protect the surface from lateraldoping from out-gassing of the buried layers. The oxidation is also usedto help define a pattern in the wafer for subsequent mask alignment,since the oxide growth rate over antimony NBL layer 136A will be fasterthan over boron PBL layer 137A.

After buried layer diffusion, the oxide is stripped off as shown in FIG.4E and an HCl acid etch is performed in-situ at the beginning ofepitaxial growth, thereby removing the top silicon layers to improveadhesion and reduce crystal defects in the epitaxial layer. The resultof the epitaxial growth is shown in FIG. 4F where epitaxial layer coversthe now expanded NBL region 136B and PBL 137B, both up-diffusing intothe epitaxial layer during its high temperature growth.

Next, as shown in FIGS. 4G and 4H. a high dose phosphorus implant 140 isintroduced through a mask 141, followed by a high dose boron implant 142through a photoresist mask 143. After a long high temperature isolationdiffusion P-type isolation region 145 connects with a portion of P-typeburied layer PBL 137C. Similarly, N-type sinker diffusion 144 connectswith buried layer NBL 136C. The depth of the diffusion and the timerequired depend on the thickness of epitaxial layer 139 and othersubsequent high temperature diffusions in the process. High temperaturediffusion also causes the buried layers to further expand laterally andup-diffuse to form 137C and 136C larger than their size in the previousprocessing step, i.e. as 137B and 136B.

Any DMOS body diffusions will also change all the junction depths andthe net epitaxial thickness above the buried layers, all makingmanufacturing processing complex and specific to a particular epitaxialthickness. Since epi thickness determines device voltage ratings, theentire process and the corresponding design rules are all voltagespecific.

Adapting Low-Temperature Modular Fabrication to High-Voltage Devices

As described previously, the problem with conventional epitaxial andhigh-temperature processes and manufacturing methods used to fabricate,isolate, and integrate high-voltage devices is that each hightemperature process causes dopant redistribution affecting everyhigh-voltage and low-voltage device. High temperature fabrication alsoprecludes the use of large diameter wafers and modern submicron waferfabs—fabs capable of high-density transistor integration, large die andhigh yields at low manufacturing costs.

What is needed is a process for integrating high-voltage and DMOStransistors with fully-isolated floating pockets of low-voltage CMOS,bipolar transistors, diodes, and passive circuit components thateliminates the need for high temperature processing and epitaxy.Ideally, such a manufacturing process should employ “as-implanted”dopant profiles—ones where the final dopant profiles remainsubstantially unaltered from their original implanted profiles by anysubsequent wafer processing steps. Ideally the process should beconstructed in a modular architecture where devices may be added oromitted and the corresponding process steps added or removed to theintegrated flow without changing the other devices available in theprocess's device arsenal.

SUMMARY OF THE INVENTION

In accordance with this invention, a series of processes are used tointegrate high-voltage and DMOS transistors with fully-isolated floatingpockets of low-voltage CMOS, bipolar transistors, diodes, and passivecircuit components. The processes eliminate the need for hightemperature processing and epitaxy and employ “as-implanted” dopantprofiles—ones where the final dopant profiles remain substantiallyunaltered from their original implanted profiles by any subsequent waferprocessing steps. Together, the processes form a modular architecturewhich allows devices to be added or omitted to the IC and thecorresponding process steps added to or removed from the integrated flowwithout the necessity of altering the processes used to produce theother devices on the IC.

Advantageously, the processes are performed on a substrate without anepitaxial layer and do not include the formation of an epitaxial layer.

Using these low-temperature processes, a number of unique high voltageand power devices may be fabricated and integrated into an IC in amodular fashion. Included are a non-isolated lateral DMOS, non-isolatedextended drain or drifted MOS devices, a lateral trench DMOS, anisolated lateral DMOS, JFET and depletion-mode devices, along with P-Ndiode clamps and rectifiers and junction termination for low-voltagecomponents floating at high voltages with respect to the substrate.

A process of fabricating the non-isolated DMOS includes the implantationof a conformal drift region through a field oxide layer; theimplantation of a drain region within the drift region at a first end ofthe field oxide layer; the formation of a gate at a second end of thefield oxide layer; and the implantation of a body region near the secondend of the field oxide layer; and the implantation of a source regionwithin the body region. The drift and body regions may be formed withchained implants to produce a non-Gaussian vertical dopant profile. Thenon-isolated DMOS may be fabricated in a drain-centric form. In oneembodiment, the field oxide layer may be omitted and the drift and bodyregions may be fabricated with a chained implant to produce anon-Gaussian vertical dopant profile. In another series of embodiments,lateral DMOS is formed with a Zener diode clamp to create a more robustavalanche-rugged device. The device may also be formed with an extendeddrain, and the gate may or may not surround the drain.

A process of fabricating a non-isolated extended drain or drifted MOSdevice may create a drain-centric device with an extended drain that isself-aligned to the gate, which may surround the drain. The device maybe formed in a non-Gaussian well. In an alternative embodiment, thedevice is asymmetric and the gate does not surround the drain. A CMOSpair may be fabricated using this asymmetric structure.

A process of fabricating a lateral trench DMOS (LTDMOS) may include theformation of a trench gate, the implantation of a drift layer which mayextend to a level near the bottom of the trench, the formation ofas-implanted body, preferably using a chain implant of varying implantenergies and doses, and the formation of source and drain regions. Thisdevice may be fabricated in a trench gate-centric form. The LTDMOS mayinclude a deep drain region which may be surrounded by a conformal driftregion. The device may be fabricated in a drain-centric form. By theproper placement of field oxide segments at the surface of thesemiconductor material, the device may include a conformal drift regionhaving deeper portions in the vicinities of the trench and drain.

A process of fabricating an isolated lateral DMOS typically includes theimplantation of a deep layer of opposite conductivity to the substrate.By implanting the deep layer through an opening in a field oxide layer,the deep layer may be in the form of a saucer, with edges that extendupward to the edges of the field oxide layer so as to form a isolatedpocket. A body region may be implanted within the isolated pocket usinga chained implant. A drift region may also be implanted into the pocket.Alternatively, the field oxide layer may be omitted, in which case thedeep layer is substantially flat. The isolated pocket may be formedusing an implanted well that extends downward from the semiconductorsurface and overlaps the deep layer. The lateral DMOS may be symmetricabout the body region.

A process of fabricating a junction field-effect transistor (JFET) mayinclude the implantation of a drift region of opposite conductivity typeto the substrate and the implantation of source, drain and body (gate)regions within the drift region. The source and drain regions are of thesame conductivity type as the drift region; the body (gate) is of thesame conductivity type as the substrate. The drain region may include adeep chained implant.

A process of fabricating a depletion-mode MOS device may include forminga gate over a semiconductor surface, implanting a drift regionself-aligned to the gate, and implanting source and drain regions. Theprocess may also include using a chained implant to form a deep drainregion. In an alternative embodiment the drift region is implanted priorto the formation of the gate and is therefore not self-aligned to thegate. In yet another embodiment a deep conformal drift region isimplanted prior to the formation of the gate. Each of the foregoingembodiments may be modified to include a subsurface shield to reduce theonset on NPN parasitic bipolar conduction and to suppress snapbackeffects. The depletion-mode device may also be fabricated in a fullyisolated form with a deep isolation layer overlapped by an annularsidewall isolation wells that may also function as a deep drain.

A process for fabricating a diode may include using multiple or chainedimplants to form the anode or cathode, thereby forming a region whereinthe deeper portions are more highly concentrated than the surfaceportions. An isolated diode may be formed by implanting a deep layer andannular wells that adjoin the deep layer and surround the anode andcathode regions.

A process of this invention may also be used to form a termination edgefor floating isolated P-type pockets to high-voltages above thesubstrate. Variations of the process include the formation of metal orpolysilicon field plates atop an interlevel dielectric or field oxidelayer. The N-well that forms the sidewall of the isolation structure mayextend laterally under the field oxide layer. Another embodimentincludes a polysilicon field plate that overlaps the edge of the N-welland has a portion extending over the field oxide layer. In anotherembodiment the termination includes a deep N-drift region connected tothe N-well and extending under the interlevel dielectric or field oxidelayer. In some embodiments the deep N layer that forms the floor of theisolation structure extends laterally beyond the isolated pocket.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G are schematic diagrams of various NMOS switch-loadtopologies used in high-voltage and power applications: FIG. 1A shows alow-side switch (LSS) with grounded body; FIG. 1B shows an isolated ordiscrete low-side switch (LSS) with an integral source-body short; FIG.1C shows a high-side switch (HSS) with a grounded body; FIG. 1D shows anisolated or discrete high-side switch (HSS) with an integral source-bodyshort; FIG. 1E shows a pass transistor with a grounded body; FIG. 1Fshows an isolated or discrete AC switch with integral source-bodyshorts; FIG. 1G shows an isolated AC switch with body bias generator

FIGS. 2A-2D illustrate various aspect of snapback in lateral MOSdevices. FIG. 2A is a cross-sectional view of a device with a schematicoverlay of a parasitic NPN; FIG. 2B illustrates the phenomenon of impactionization in the device shown in FIG. 2A; FIG. 2C is an I_(D)-V_(DS)graph of the electrical characteristics of the device; FIG. 2D is aschematic representation of the device.

FIGS. 3A-3G illustrate a conventional process for fabricating lateralDMOS devices. FIG. 3A shows a cross-sectional view of the completeddevice; FIG. 3B illustrates a self-aligned body implant; FIG. 3Cillustrates a body diffusion; FIG. 3D illustrates the formation of aself-aligned body using a tilt implant; FIG. 3E illustrates anon-self-aligned body implant; FIG. 3F illustrates the stages of a bodydiffusion; FIG. 3G illustrates the formation of a non-self-aligned gate.

FIGS. 4A-4I are a series of cross-sectional views illustrating aconventional process for a high-temperature junction isolation of anepitaxial layer.

FIGS. 5A-5C are a series of cross-sectional views illustrating alow-temperature fabrication of a non-isolated lateral DMOS.

FIG. 6 illustrates a cross-sectional view of a non-isolated lateral DMOSwith a non-Gaussian P-type well and a conformal N-type drift region.

FIG. 7 is a cross-sectional view of a non-isolated lateral DMOS with anon-Gaussian P-type well and a drift region comprising a conformalN-type chain-implanted well.

FIG. 8 is a cross-sectional view of a non-isolated lateral DMOS with anon-Gaussian P-type well and a uniform N-type drift region.

FIG. 9 is a cross-sectional view of a non-isolated lateral DMOS with anon-Gaussian P-type well and a drift region comprising a uniform N-typechain-implanted well.

FIGS. 10A-10D are cross-sectional views of a non-isolated lateral DMOSwith a non-Gaussian P-type well as DMOS body and an avalanche clampeddrain. In FIG. 10A, the DMOS has a shallow N− drift drain region. InFIG. 10B, the DMOS has a uniform N-type deep drift region as a drainextension. In FIG. 10C, the DMOS has a conformal N-type drift region asa drain extension. In FIG. 10D, the DMOS has a conformal N-type well asdrain extension.

FIGS. 11A-11D illustrate aspects of avalanche clamping of a non-isolatedlateral DMOS using a P-body (or P-base). FIG. 11A is a cross-sectionalview of the device. FIG. 11B is a schematic representation of thedevice. FIG. 11C shows the I_(D)-V_(DS) electrical characteristics ofthe device. FIG. 11D shows the equipotential distribution in the deviceat high voltages.

FIG. 12 is a cross-sectional view of a non-isolated extended-drain PMOSwith a graded drain.

FIG. 13 is a cross-sectional view of a non-isolated extended-drain NMOSwith a graded drain.

FIG. 14 is a cross sectional view of a non-isolated extended-drain CMOS.

FIGS. 15A-15C are a series of cross-sectional views illustrating thefabrication of a lateral trench DMOS.

FIGS. 16A and 16B are cross-sectional and top views, respectively,showing the construction of a trench lateral DMOS with a uniform deepN-type drifted drain.

FIGS. 17A-17F illustrate variants of a trench lateral DMOS. FIG. 17Ashows the P-body juxtaposing the N-well drain. FIG. 17B shows theminimum field oxide spacing of the P-body and the N-well drain. FIG. 17Cshows an extended uniform drift region. FIG. 17D shows an extendedconformal drift region. FIG. 17E shows an N-well drain overlapping aP-body. FIG. 17F shows a device having no N-well drain.

FIGS. 18A-18C illustrate the construction of a trench lateral DMOSsurrounded by a drain. FIG. 18A is a cross-sectional view. FIG. 18B is aplan view of a device with a reduced body width. FIG. 18C is a plan viewof a device with staggered source-body contacts.

FIGS. 19A-19C are a series of cross-sectional views illustrating thefabrication of an isolated lateral DMOS.

FIG. 20 is a cross-sectional view of an isolated lateral DMOS with aconformal deep drift drain region.

FIG. 21 is a cross-sectional view of an isolated lateral DMOS with achained implanted N-well as a drifted drain region.

FIG. 22 is a cross-sectional view of an isolated lateral DMOS with ashallow N-drift drain region.

FIG. 23 is a cross-sectional view of a high voltage JFET with a uniformdeep drifted drain region.

FIG. 24 is a cross-sectional view of a depletion-mode NMOS with ashallow LDD.

FIG. 25 is a cross-sectional view of a depletion-mode NMOS with auniform deep drifted drain region.

FIG. 26 is a cross-sectional view of a depletion-mode NMOS with aconformal deep drifted drain region.

FIGS. 27A-27C are cross-sectional views of variants of a depletion-modeNMOS with a subsurface source shield. FIG. 27A shows a device with ashallow LDD drain. FIG. 27B shows a device with a uniform deep drifteddrain. FIG. 27C shows a device with a conformal deep drifted drain.

FIG. 28 is a cross-sectional view of and isolated depletion-mode NMOSwith a shallow LDD.

FIGS. 29A-29E are cross-sectional views of various Zener clampingdiodes. FIG. 29A shows a device with a non-isolated N+ to P-well and anN+ to P-base or P-body. FIG. 29B shows a device with an isolated N+ toP-base or P-body. FIG. 29C shows a device with an isolated N+ to P-well.FIG. 29D shows a device with a multi-stripe isolated N-well to P-wellburied Zener. FIG. 29E shows a device with an isolated P+ to N-base.

FIGS. 30A-30K are cross-sectional views of high-voltage terminations ofan isolated P-type pocket.

DESCRIPTION OF THE INVENTION

U.S. Pat. No. 6,855,985 describes an all low-temperature fabricationmethod using as-implanted junction isolation structures. This methodemploys high-energy and chain implants with dopant implanted throughcontoured oxides to achieve fully-isolated bipolar, CMOS and DMOSdevices without the need for isolation diffusions, epitaxy or hightemperature processes.

The subject matter in this application is related to theabove-referenced patent and focuses on the design and integration ofvarious kinds of new or improved high-voltage and DMOS devices, snapbackprevention, isolated clamping diodes and rectifiers, and methods tofloat low-voltage devices in isolated pockets to high voltages above thesubstrate potential.

The low-temperature fabrication of the high-voltage devices describedherein are compatible with the modular low-temperature fabricationmethods described in the aforementioned patents and patent applications,but are not necessarily limited to modular process architectures.

Wafer Fabrication

Except as specifically noted, the fabrication of the high-voltage andpower devices described herein utilizes the same process sequences thatare described in the above referenced patents. A brief summary of thebasic process flow includes

-   -   Field oxide formation    -   Trench and trench gate formation including planarization    -   High-energy implanted deep drift layer (ND) formation    -   Chain-implant trench DMOS body (P-Body) formation    -   Phosphorus high-energy implanted floor isolation (DN) formation    -   First chain-implanted non-Gaussian N-well (NW1/NW1B) formation    -   First chain-implanted non-Gaussian P-well (PW1/PW1B) formation    -   Second chain-implanted non-Gaussian N-well (NW2/NW2B) formation    -   Second chain-implanted non-Gaussian P-well (PW2/PW2B) formation    -   Dual gate oxide and gate electrode formation    -   N-base implant    -   P-base implant    -   First N-LDD implant (NLDD1)    -   First P-LDD implant (PLDD1)    -   Second N-LDD implant (NLDD2)    -   Second P-LDD implant (PLDD2)    -   Sidewall spacer formation    -   ESD implant    -   N+ implant    -   P+ implant    -   Rapid thermal anneal (RTA implant activation)    -   Multilayer metal interconnect process    -   Passivation

Since the process as described utilizes as-implanted dopant profileswith little or no dopant redistribution, implants may be performed invirtually any order except that it is preferred that the P-well andN-well implantation precede gate formation, the trench gate formationprecede DMOS body implantation, N-LDD and P-LDD implants follow gateformation but precede sidewall spacer formation, and N+ and P+ implantsfollow sidewall spacer formation. This process flow is designed to bemodular, so it is possible to eliminate one or more process steps forfabrication of a given IC, depending on which set of devices arerequired for that IC design.

By way of example, Table 1 summarizes a preferred embodiment and apreferred range of conditions for the implants described in thisapplication:

TABLE 1 Preferred Embodiment Preferred Range Implant (Species) (Energy,Dose) (Energy, Dose) DN (P⁺) E = 2.0 MeV, E = 1.0 MeV to 3.0 keV, Q =2E13 cm⁻² Q = 1E12 to 1E14 cm⁻² ND deep drift (P⁺) E = 800 keV, E = 400keV to 1.2 MeV, Q = 2E12 cm⁻² Q = 5E11 to 5E12 cm⁻² E = 600 keV, E = 300keV to 900 keV, Q = 2E12 cm⁻² Q = 5E11 to 5E12 cm⁻² P-body (B⁺) E = 120keV, E = 60 keV to 180 keV, Q = 2E12 cm⁻² Q = 5E11 to 5E12 cm⁻² E = 80keV, E = 40 keV to 120 keV, Q = 4E12 cm⁻² Q = 1E12 to 1E13 cm⁻² 1stP-well + (B⁺) E = 240 keV, E = 120 keV to 360 keV, Q = 1E13 cm⁻² Q =5E12 to 5E13 cm⁻² E = 120 keV, E = 60 keV to 180 keV, Q = 6E12 cm⁻² Q =1E12 to 1E13 cm⁻² 1st N-well + (P⁺) E = 460 keV, E = 230 keV to 690 keV,Q = 5E12 cm⁻² Q = 1E12 to 1E13 cm⁻² E = 160 keV, E = 80 keV to 240 keV,Q = 1E12 cm⁻² Q = 5E11 to 5E12 cm⁻² 2nd P-well + (B⁺) E = 460 keV, E =230 keV to 690 keV, Q = 1E13 cm⁻² Q = 5E12 to 5E13 cm⁻² E = 160 keV, E =80 keV to 240 keV, Q = 1E12 cm⁻² Q = 5E11 to 5E12 cm⁻² 2nd N-well + (P⁺)E = 950 keV, E = 500 keV to 1.5 MeV, Q = 1E13 cm⁻² Q = 5E12 to 5E13 cm⁻²E = 260 keV, E = 130 keV to 390 keV, Q = 1E12 cm⁻² Q = 5E11 to 5E12 cm⁻²N-base (P⁺) E = 300 keV, E = 150 keV to 450 keV, Q = 2E12 cm⁻² Q = 5E11to 5E12 cm⁻² E = 120 keV, E = 60 keV to 180 keV, Q = 9E12 cm⁻² Q = 5E12to 5E13 cm⁻² P-base (B⁺) E = 240 keV, E = 120 keV to 360 keV, Q = 6E12cm⁻² Q = 1E12 to 1E13 cm⁻² E = 100 keV, E = 50 keV to 150 keV, Q = 6E12cm⁻² Q = 1E12 to 1E13 cm⁻² NLDD1 (P⁺) E = 80 keV, E = 40 keV to 160 keV,Q = 2E13 cm⁻² Q = 5E12 to 5E13 cm⁻² PLDD1 (BF₂ ⁺) E = 80 keV, E = 40 keVto 160 keV, Q = 2E12 cm⁻² Q = 5E11 to 5E12 cm⁻² NLDD2 (P⁺) E = 80 keV, E= 40 keV to 160 keV, Q = 6E12 cm⁻² Q = 1E12 to 1E13 cm⁻² PLDD2 (BF₂ ⁺) E= 100 keV, E = 50 keV to 150 keV, Q = 3E12 cm⁻² Q = 1E12 to 1E13 cm⁻² N+(As⁺) E = 30 keV, E = 20 keV to 60 keV, Q = 5E15 cm⁻² Q = 1E15 to 1E16cm⁻² P+ (BF₂ ⁺) E = 30 keV, E = 20 keV to 60 keV, Q = 3E15 cm⁻² Q = 1E15to 1E16 cm⁻²

Several of the above implants are potentially usable to form the driftregions of high voltage devices because the total implant dose issufficiently low to support two-sided depletion spreading, and in somecases to allow complete depletion of the layer prior to the onset ofavalanche breakdown. The phenomenon whereby surface electric fields in adevice are reduced at high voltages by fully depleting a region oflimited implant dose (or charge) is also referred to as “RESURF”, anacronym for reduced surface fields. Historically, the term RESURF wasused for epitaxial layers of limited charge while LDD, drift region, ordrain extension referred to implanted layers. No distinction is madeherein between the advantages of shallow LDD versus deep drift regionsexcept that shallow LDD regions are typically self aligned to a MOS gatewhile deep high energy implanted drift regions (such as the ND implant)typically precede gate formation.

In one embodiment of this invention, the first and second wells in theabove table refer to the 5V and 12V P-wells and N-wells used forfabricating 5V and 12V CMOS. The terms 5V and 12V are not used to belimiting but only describe two different P-type well concentrations andtwo-different N-type well concentrations, e.g. 3V and 15V, or 12V and30V, 1.5V and 3V, etc. In general the lower voltage wells tend to bemore heavily doped than the higher voltage wells, especially near thesilicon surface, but with non-Gaussian dopant profiles comprising acombination of various implants differing in dose and energy, i.e. achain implant, the lower voltage wells are not necessarily the higher inpeak concentration, in average concentration, or in total implantedcharge (dose). Higher voltage wells also tend to be deeper than N-wellsoptimized for low voltage devices. In one embodiment for example, theN-well and P-well for 5V CMOS utilize implants with a mean projectedrange of 0.4 to 0.5 microns, while the wells needed for 12V CMOS have amean projected range between 0.7 to 1.1 microns in depth in activeareas. The depth under the field oxide is reduced roughly by thethickness of the layer during implantation. The deep N-type drift mayhave a depth in active areas similar to 12V wells or slightly deeper.

The applicability of such a well as a drift region or extended draindepends on the two-dimensional electric field distribution at the onsetof avalanche. In non-Gaussian and retrograde dopant profiles, thesurface electric field and breakdown voltage does not trackconcentration as simply as it does in conventional diffused wellscomprising purely Gaussian dopant profiles.

Consistent with this observation, in the subsequent figures each well isdescribed by two graphical elements, a top portion e.g. labeled PW1 forfirst P-well; and a buried or deeper portion labeled PW1B for firstburied P-well. To be buried beneath the well's top portion, the buriedportion is implanted at a higher energy to reach a greater depth.Typically the buried portion of the well will also use a higher implantdose and exhibit a higher peak concentration than the surface portion,i.e. the dopant profile will be retrograde—more concentrated in the bulkthan at the surface, although it is not required for manufacturing'ssake. Retrograde profiles cannot be produced using conventional diffusedjunctions without the need for expensive epitaxial depositions and hightemperature buried layer diffusions described previously.

The wells may in fact comprise any number of implants of differingenergies and implant doses, to create arbitrary dopant profiles usefulin both low-voltage CMOS and in high voltage device fabrication. For thesake of clarity we refer to the top and bottom well portions distinctlybut collectively, we describe their combination as one well. Forexample, PW1 and PW1B together comprise a first P-well (e.g. for 5Vdevices), while PW2 and PW2B together comprise a second well foroperation at higher voltages. In general the second well, being morelightly doped, is likely the better candidate to operate as a highvoltage drift region, but may in fact be worse if its surfaceconcentration is higher. In principle, designing the peak concentrationand electric fields within a high voltage device to occur deeper in thesemiconductor, away from the surface, should result in a more robustdevice useful at higher voltages.

Using the aforementioned process architecture, a number of unique highvoltage and power devices may be fabricated and integrated into an IC ina modular fashion. These new high voltage devices include a non-isolatedlateral DMOS, non-isolated extended drain or drifted MOS devices, alateral trench DMOS, an isolated lateral DMOS, JFET and depletion-modedevices, along with P-N diode clamps and rectifiers and junctiontermination for low-voltage components floating at high voltages withrespect to the substrate.

Non-Isolated Lateral DMOS

One type of high-voltage transistor fabricated using the low-temperaturefabrication methods described herein is a non-isolated lateral DMOStransistor.

The fabrication of a non-isolated lateral DMOS transistor 200 isillustrated in cross section in FIGS. 5A-5C. The process includes ahigh-energy ion implantation of a lightly-doped N-type drift regionthrough a contouring oxide. As shown in FIG. 5A, an implant contouringfield oxide layer 204 formed in substrate 201 using LOCOS (for example),is selectively masked with photoresist 202 and implanted with ahigh-energy phosphorus drift implant to form a non-uniform, conformaldrift region 203, with a shallower portion 203A beneath oxide 204 anddeeper portions 203B and 203C beneath active areas not covered by fieldoxide layer 204. (Note: As used herein, the term “conformal” refers to aregion or layer of dopant (a) that is formed by implantation through alayer (often an oxide layer) at the surface of the semiconductormaterial, and (b) whose vertical thickness and/or depth in thesemiconductor material vary in accordance with the thickness and/orother features of the surface layer, including any openings formed inthe surface layer.) No implant penetrates mask 202. The total integratedcharge Q of deeper active areas 203B and 203C is greater than theshallower drift region 203A. The total charge present in the silicon isgiven by

Q = ∫_(x₁)^(x_(j))N(x)𝕕xwhere in the case of active areas x₁=0, i.e. the silicon surface. Forimplants under the field oxide, x₁ is the interface between the fieldoxide and the underlying silicon. Since the implant is the same in bothregions, the dopant in the silicon under the field oxide is less than inthe active areas. Beneath the transition area from no oxide to fullthickness, i.e. the bird's beak area, the total drift charge is graded,a natural artifact of the disclosed manufacturing process.

As shown in FIG. 5B, a P-type well 206 is introduced into an areaadjacent drift region 203 by ion implantation through photoresist mask205. In the final device, this P-well will serve as the body of thelateral DMOS but because it precedes the gate formation, it is notself-aligned to the transistor's gate. Unlike a conventional diffusedwell which has its peak concentration near the surface and amonotonically decreasing concentration with increasing depth, P-typewell 206 is formed by high energy ion implantation of boron, andpreferably by a boron chain implant comprising a series of boronimplants varying in dose and energy. The chain implant, while it maycomprise any number of implants, is graphically represented in thedrawing by two regions—a surface layer PW1, and a subsurface layer PW1B,formed by ion implantation through a single mask and without the use ofepitaxy. In a preferred embodiment the deeper layer is more highlyconcentrated than the surface well.

In FIG. 5C, N-well 207 is introduced into the drain region of the DMOSinside N-type drift region 203C to reduce the transistor's drainresistance and to further shape the electric field in the drain toreduce pre-avalanche impact ionization. To avoid dopant redistributionfrom high temperature diffusion, N-well 207 is formed using high energyion implantation of phosphorus, and preferably by a phosphorus chainimplant comprising a series of phosphorus implants varying in dose andenergy. The chain implant, while it may comprise any number of implants,is graphically represented in the drawing by two regions—a surface layerNW1, and a subsurface layer NW1B, formed by ion implantation through asingle mask and without the use of epitaxy. In a preferred embodimentthe deeper layer is more highly concentrated than the surface well.

While the cross section illustrates a first P-well 206 and a firstN-well 207 which may for example represent P-well PW1 and N-well NW1used for 5V CMOS, it is also possible to utilize a second P-well and asecond N-well such as PW2 and NW2 optimized for other voltage CMOS, e.g.12V or 20V devices.

After well formation, gate oxide 209 is grown, a polysilicon layer isdeposited and patterned to form gate 208. This gate is preferablypositioned above a portion of P-type well 206 and a portion of driftlayer 203 to insure proper transistor action.

Aside from its all low temperature fabrication and integration into afully-modular process architecture, N-channel lateral DMOS device 200offers other advantages over conventionally fabricated lateral DMOSincluding reduced surface electric fields under the gate correspondingto less hot carrier generation resulting from its lower well surfaceconcentration; enhanced subsurface electric fields from theheavily-doped deeper portion of the P-well 206 forcing avalanchebreakdown into the bulk and away from the semiconductor's surface; and,improved gain and lower on-resistance resulting from a shorter length ofgate 208 located atop active areas, the shorter gate made possible bythe nearly vertical sidewall junction of chain-implanted P-well 206. Incontrast, diffused well DMOS require longer gates to cover the largerlateral dimensions of a high-temperature diffused well.

An example of drain-centric lateral DMOS 220 using a non-Gaussian P-typewell and conformal N-type drift is shown in cross section in FIG. 6. Thedevice comprises drift 223 conforming to field oxide layer 222 withN-well drain 235, N+ drain 236, N+ source 228 with lightly dopedextension 230, P+ body contact 229, polysilicon gate 226 with overlyingsilicide 227 formed atop gate oxide 225, interlevel dielectric (ILD)233, metal interconnect 231 with contact barrier metal 232. The driftlength L_(D1), as measured from the edge of the active gate to the edgeof active N-well NW1 can be adjusted to select the breakdown voltage ofthe device without substantially altering the manufacturing process. Inthis device the DMOS body is formed by P-wells 224A and 224B which maycomprise a stripe geometry or an annular geometry, enclosing the drainon all or a few sides. (Note: As used herein, the term “annular” refersto a doped region or other structure that laterally surrounds a featurein an IC chip. The annular region or structure may be circular, square,rectangular, polygonal or any other shape, and the annular region orother structure may or may not be in contact with the feature that itlaterally surrounds.) The P-well as shown comprising a non-Gaussian ornon-monotonic profile represented by a top portion PW1 and a subsurfaceportion PW1B, where in a preferred embodiment PW1B is more heavily dopedthan PW1, and in an alternative embodiment P-wells 224A and 224Bcomprise a series of chained implants differing in dose and energy.

FIG. 7 illustrates in cross section a non-isolated lateral DMOS 240 withnon-Gaussian P-type well and drift comprising a conformal N-typechain-implanted well. The device comprises drift 243 conforming to fieldoxide layer 242, with N+ drain 256, N+ source 248 with lightly dopedextension 250, P+ body contact 249, polysilicon gate 246 with overlyingsilicide 247 formed atop gate oxide 245, ILD 253, metal interconnect 251with contact barrier metal 252. In this device the DMOS body is formedby P-wells 244A and 244B which may comprise a stripe geometry orannular, enclosing the drain on all or a few sides. Each of the P-wells244A and 244B is shown comprising a non-Gaussian or non-monotonicprofile represented by a top portion PW1 and a subsurface portion PW1B,where in a preferred embodiment PW1B is more heavily doped than PW1, andin an alternative embodiment P-wells 244A and 244B comprise a series ofchained implants differing in dose and energy.

The drift 243, constructed of a chain implanted N-well, comprises a deepportion NW1B and a shallower portion NW1. The shallower portion NW1 islocated in active areas such as 243B. Non-active areas such as 243Ainclude only the deep portion NW1B, reducing series drain resistancewithout increasing the electric field under gate oxide 245. The driftlength L_(D1), measured from the edge of the active gate to the edge ofactive N-well NW1, can be adjusted to select the breakdown voltage ofthe device without substantially altering the manufacturing process. Ingeneral, a higher total integrated charge in the drift is preferable inlow-voltage drifted-drain devices. A first N-well and P-well may besubstituted by a second N-well and P-well in varying combinationsdepending on the dopant profiles and the intended voltage range of thedevice.

FIG. 8 illustrates in cross section a non-isolated lateral DMOS 260comprising a non-Gaussian P-type well and an N-type drift formed inactive regions only (not under field oxide). The device comprises drift263, N-well drain 265, N+ drain 276, N+ source 268 with a lightly dopedextension 270, P+ body contact 269, polysilicon gate 266 with overlyingsilicide 267 formed atop gate oxide 265, ILD 273, metal interconnect 271with contact barrier metal 272. In this device the DMOS body is formedby P-wells 264A and 264B which may comprise a stripe geometry or anannular geometry, enclosing the drain on all or a few sides. The P-wellis shown comprising a non-Gaussian or non-monotonic profile representedby a top portion PW1 and a subsurface portion PW1B, where in a preferredembodiment PW1B is more heavily doped than PW1, and in an alternativeembodiment P-wells 264A and 264B comprise a series of chained implantsdiffering in dose and energy.

The drift 263, constructed of a high energy implanted drift layer ND, isoptimized for avalanche and breakdown characteristics. Multiple implantswith various doses and energies may be combined to form the drift layerND. The drift length L_(D1), as measured from the edge of the gate tothe edge of N-well NW1 can be adjusted to select the breakdown voltageof the device without substantially altering the manufacturing process.In general, a higher total integrated charge in the drift is preferablein low-voltage drifted-drain devices. A first N-well and P-well may besubstituted by a second N-well and P-well in varying combinationsdepending on the dopant profiles and the intended voltage range of thedevice.

The source metal interconnect 271A and 271C is shown extending over andbeyond the gate 266 to overlap a portion of drift 263. This optionallayout allows the source metal to serve as a field plate to reduce theelectric field crowding near the end of the gate, thereby increasing thebreakdown voltage of DMOS 260. Note that this layout is optional and maybe applied to all other DMOS devices in this description, as well.

FIG. 9 illustrates in cross section a non-isolated lateral DMOS 280 witha non-Gaussian P-type well and a drift region comprising uniform N-typechain-implanted well. The device utilizes a drift comprising achain-implanted N-well 283 formed in active areas only (not under fieldoxide), N+ drain 296, N+ source 288 with a lightly doped extension 290,P+ body contact 289, polysilicon gate 286 with overlying silicide 287formed atop gate oxide 285, ILD 282, metal interconnect 291 with contactbarrier metal 292. In this device the DMOS body is formed by P-wells284A and 284B which may comprise a stripe geometry or an annulargeometry, enclosing the drain on all or a few sides. Each of P-wells284A and 284B is shown comprising a non-Gaussian or non-monotonicprofile represented by a top portion PW1 and a subsurface portion PW1B,where in a preferred embodiment PW1B is more heavily doped than PW1, andin an alternative embodiment P-wells 284A and 284B comprise a series ofchained implants differing in dose and energy.

The drift, constructed of a chain-implanted N-well 283, comprises anon-Gaussian or non-monotonic profile represented by a top portion NW1and a subsurface portion NW1B, where in a preferred embodiment NW1B ismore heavily doped than NW1, and in an alternative embodiment N-well 283comprises a chain-implant constructed using a series of implantsdiffering in dose and energy.

The drift length L_(D1), measured from the edge of the gate 286 to theedge of the N+ drain 296, can be adjusted to select the breakdownvoltage of the device without substantially altering the manufacturingprocess. In general higher total integrated charge in the drift ispreferable in low-voltage drifted-drain devices. A first N-well andP-well may be substituted by a second N-well and P-well in varyingcombinations depending on the dopant profiles and the intended voltagerange of the device.

In FIG. 9, the N+ source 288 is shown separated from P+ body contact289, and source metal interconnect 291B is shown separated from the bodymetal interconnect 291A. This optional layout allows the source voltageto float above the body voltage up within the limitation of the sourceto body breakdown voltage (typically several volts) and may be used toallow sensing of the current flowing through DMOS 280. Note that thislayout is optional and may be applied to all other DMOS devices in thisdescription, as well.

Note that many features shown in FIGS. 6-9 are for illustrative purposesonly, and that modifications to these structures are within the scope ofthis invention. The lightly doped source extension 230 of FIG. 6, forexample, is not necessary for operation of the LDMOS devices, andalternative embodiments in which this region is replaced by a moreheavily-doped source extension may actually provide better on-statecharacteristics. While only a single level of metal interconnect isshown in these examples, preferred embodiments will of course compriseadditional levels of metallization that serve to lower the on-resistanceof the DMOS devices and could also form source and/or drain fieldplates, internal bussing for the gate, etc. The metallization layer isshown extending into the ILD layer, but other preferred embodiments willuse a metal plug (e.g. tungsten) to fill the contact hole in the ILDlayer, and a planar metallization layer on top of the ILD layer. Thefield oxide layers are shown to comprise LOCOS, but alternate structuressuch as deposited or grown and etched back oxide, recessed oxide, andnon-oxide dielectric materials may also be employed.

FIGS. 10A-10D illustrate a modifications of the aforementioned lateralDMOS with a Zener diode clamp. This produces a more robustavalanche-rugged device, even though the breakdown voltage is decreasedby the presence of the clamp. The increased ruggedness results fromforcing avalanche into the bulk silicon beneath the drain, far away fromthe MOS gate.

In the lateral DMOS 300 of FIG. 10A, for example, the Zener clampcomprises P-well 319 located beneath N+ drain 311. The same P-wellimplant steps may be used to form P-well 319 and P-well 302, thenon-Gaussian body of the lateral DMOS 300. Alternatively, differentP-well implant steps may be used to form P-wells 319 and 302. P-wells319 and 302 as shown comprise a non-Gaussian or non-monotonic profile,represented by a top portion PW1 and a subsurface portion PW1B. In apreferred embodiment PW1B is more heavily doped than PW1, and in analternative embodiment comprises a chain-implant of multiple implantsdiffering in dose and energy. The drain extension in this devicecomprises a shallow N− drift 310 which may also be used for 12V NMOSdevices. The remaining elements of the device are similar to theaforementioned lateral DMOS, including N+ source 304, P+ body contact303, gate oxide 307, poly gate 308, silicide 309, sidewall spacer 306,N− source extension 305, ILD 315, barrier metal 312, and metalinterconnect 313.

An avalanche clamped lateral DMOS 320 is shown in FIG. 10B with ahigh-energy implanted drift 330 and P-well clamp 334, similar to avoltage clamped version of DMOS 260 of FIG. 8.

An avalanche clamped lateral DMOS 340 is shown in FIG. 10C with aconformal high-energy implanted drift 350 and P-well clamp 342B, similarto a voltage clamped version of DMOS 220 of FIG. 6.

Another avalanche clamped lateral DMOS 340 is shown in FIG. 10D usingchain-implanted N-well 370A as a drift with P-well clamp 362B, similarto a voltage clamped version of DMOS 240 of FIG. 7.

It should be noted that the devices of FIGS. 10A-10D are shown assymmetrical, with the centerline of the device at the right side of thedrawing. This is not required, however.

Drain voltage clamping can also be accomplished using a shallower P-typeregion than a P-well. In lateral DMOS 380 of FIG. 11A, for example, theZener clamp comprises P-base or P-body 394 located beneath N+ drain 393.P-well 382 forms the body of the lateral DMOS 380, comprising anon-Gaussian or non-monotonic profile represented by a top portion PW1and a subsurface portion PW1B. In a preferred embodiment, PW1B is moreheavily doped than PW1, and in an alternative embodiment comprises achain-implant of multiple implants differing in dose and energy. Thedrain extension in this device comprises a shallow N− drift region 392which may also be used for 12V NMOS devices. The remaining elements ofthe device are similar to those in the aforementioned lateral DMOSdevices, including N+ source 384, P+ body contact 383, gate oxide 387,gate 388, silicide 389, sidewall spacer 386; N− source extension 385,ILD 391, barrier metal 396, and metal interconnect 395.

Subcircuit schematic 400 of FIG. 11B illustrates the concept ofintegrated lateral DMOS voltage clamping schematically, where NMOS 401illustrates the N-channel DMOS, diode 402 represents the BV_(DSS) of theunclamped transistor, and Zener diode 403 illustrates the integrateddrain voltage clamp.

FIG. 11C illustrates the I_(D)-V_(DS) characteristics 410 of the voltageclamped lateral DMOS. The family of curves 412, 413, 414, and 415represent increasing drain current corresponding to increasing gatedrives. At higher current the maximum sustained voltage BV_(CER) isshown by curve 416, virtually independent of gate drive. This voltage issubstantially below the off-state breakdown BV_(DSS) shown by curve 411.Snapback from BV_(DSS) breakdown 411 to the lower BV_(CER) value 416 canresult in excess current and device damage. To absolutely avoidsnapback, the Zener breakdown voltage BV_(Z2) shown as curve 418 must beset below BV_(CER) 416. This degree of voltage clamping may overly limitthe operating voltage range of a voltage clamped lateral DMOS. Notingthat curve 413 shows a substantial negative resistance but requires acertain current level I_(D1) to invoke snapback, a clamp voltageBV_(Z1), shown by curve 417 lower than BV_(DSS) but greater thanBV_(CER) may be sufficient to achieve robust operation withoutsubstantially restricting the operating voltage range.

FIG. 11D is a simplified cross-sectional view 430 which illustrates theclamping action of the drain voltage clamp and its effect on theequipotential lines 438. In this device, the applied voltage on N+ drain439 causes N− drift 440 to deplete allowing equipotential lines 438 tospread across the drift region 435 in voltage increments 0, V₁, V₂, andV₃, thereby maintaining a low electric field near gate oxide 432 andgate 433. The action of P-type layer 437 is to increase the electricfield beneath drain 439, squeezing the equipotential lines 438 togetherand forcing breakdown in this area, which is far away from gate 433.

In summary, the drain voltage clamp methods described herein improvelateral DMOS avalanche capability by moving the breakdown location awayfrom the gate, reducing device breakdown to improve the survival rate ofdevices subjected to EOS.

Non-Isolated Extended-Drain MOS

The drain avalanche clamp concept can also be applied to devices otherthan lateral DMOS, including lightly-doped-drain (LDD) drain-extendedMOS transistors. In such devices, the length of the drain extension or“drift” L_(D) is generally longer than that of the gate's sidewallspacer, typically from one-half microns up to tens-of-microns in length.Unlike a lateral DMOS, where a more heavily doped well or body surroundsthe source extending beneath the gate, non-DMOS devices employ alaterally uniform well concentration, at least within active regions.The P-well and N-well doping profiles in the devices described hereinare as-implanted, and not produced through conventional means requiringlong high-temperature diffusions. The vertical dopant profile in thedevices, i.e. perpendicular to the wafer's surface, may thereforecomprise non-Gaussian and non-monotonic profiles used to optimize bothon-state conduction and off-state blocking characteristics.

In this section, the title “non-isolated” refers to the absence of ahigh-energy implanted DN layer in the device—a dedicated implant usedfor forming floor isolation beneath one or more devices. Without the DNimplant, any P-well is electrically shorted to the P-type substrate,meaning the body or channel of all non-isolated NMOS transistors isnecessarily grounded. P-channel devices on the other hand, are formed inN-wells and are naturally self-isolated without the need for the DNlayer. But because the subsurface portion of an N-well is typically notas heavily concentrated as the DN layer, the ability of the N-well toprevent substrate current resulting from parasitic PNP conduction(should the P-type drain become forward biased to the N-well) is not asgood as if the DN layer surrounds the PMOS N-well.

FIG. 12 shows a cross-sectional view of a non-isolated extended-drainPMOS 450. The device as illustrated is symmetrical anddrain-centric—meaning that P+ drain 463 is surrounded by gate 458(including silicide 459), source 454, and N+ well-contact 453 on bothsides as shown. The device may be constructed using stripe geometry orit may constitute a fully-enclosed rectangular or polygonal shape. Thegate oxide 457 may comprise a first thin gate oxide or a thicker gateoxide for higher voltage devices. The drift length L_(D1) ofself-aligned P− drift layer 462 is determined by the distance from gate458 to P+ drain 463, not by sidewall spacer 460. As a consequence of thepresence of sidewall spacer 460, a lightly doped source region 455 isformed. This region may be implanted using an existing Ldd implantcompatible with low voltage PMOS devices, or it may use a dedicatedimplant that is optimized for the PMOS 450. Contact through ILD 461 ismade using metal interconnect 465 with an underlying barrier metal 464.

As shown, the N-well 452A,452B comprises a non-Gaussian or non-monotonicprofile, represented by a top portion NW1 and a subsurface portion NW1B.In a preferred embodiment, NW1B is more heavily doped than NW1, and inan alternative embodiment N-well 452A,452B comprises a series of chainedimplants differing in dose and energy. Since the N-well 452A,452B isformed after field oxide layer 456, its junction depth under the fieldoxide is shallower, as shown by region 452B, and region 452B maysubstantially comprise only the buried portion NW1B of the N-well. Asecond well NW2 with buried portion NW2B may be used to substitute thefirst N-well.

Drain extension or drift region 462 comprises a shallow implantpreferably formed after gate 458 and field oxide layer 456, andtherefore being fully self-aligned to these layers. As shown, the driftregion 462 is surrounded by gate 458 and never touches or abuts fieldoxide layer 456.

An optional PB layer 466, comprising either a P-body implant, a P-baseimplant or another dedicated implant, is introduced surrounding P+ drain463 to reduce the surface electric field surrounding the drain bygrading the concentration. In addition to reducing the surface electricfield, it may also improve the transistor's avalanche-ruggedness bylowering the drain breakdown through subsurface avalanche. In PMOS 450,this bulk avalanche is represented schematically as diode 469, a voltageclamp comprising P+ drain 463, PB layer 466, and N-well 452A.

FIG. 13 illustrates a cross-sectional view of a non-isolatedextended-drain NMOS 470 that is analogous to PMOS 450. NMOS 470 asillustrated is symmetric and drain-centric; meaning N+ drain 483 issurrounded on both sides by gate 478 (including silicide 479), source474, and N+ well-contact 473. The device may be constructed using stripegeometry or it may constitute a fully-enclosed rectangular or polygonalshape. The gate oxide 477 may comprise a first thin gate oxide or athicker gate oxide for higher voltage devices. The length L_(D1) ofself-aligned N− drift region 482 is determined by the distance of gate478 to N+ drain 483, not by sidewall spacer 480. As a consequence of thepresence of sidewall spacer 480, a lightly doped source region 475 isformed. Lightly-doped source region 475 may be implanted using anexisting Ldd implant compatible with low voltage NMOS devices, or it mayuse a dedicated implant that is optimized for the NMOS 470. Contactthrough ILD 481 is made using metal interconnect 485 with an underlyingbarrier metal 484.

As shown, P-well 472A,472B comprises a non-Gaussian or non-monotonicprofile, represented by a top portion PW1 and a subsurface portion PW1B.In a preferred embodiment PW1B is more heavily doped than PW1, and in analternative embodiment P-well 472A,472B comprises a series of chainedimplants differing in dose and energy. Since P-well 472A,472B is formedafter field oxide layer 476, its junction depth under field oxide layer476, as in region 472B, is shallow and may substantially comprise onlythe buried portion PW1B of the P-well. A second well PW2 with buriedportion PW2B may be used to substitute the first P-well.

Drain extension or drift region 482 comprises a shallow implantpreferably formed after gate 478 and field oxide layer 476, andtherefore being fully self-aligned to these layers. In the device shownthe drift region 482 is surrounded by gate 478 and never touches orabuts field oxide layer 476.

An optional NB layer 486 comprising either an N-body implant, an N-baseimplant or another dedicated implant, is introduced surrounding N+ drain483 to reduce the surface electric field surrounding the drain bygrading the concentration. In addition to reducing the surface electricfield, it may also improve the transistor's avalanche-ruggedness bylowering the drain breakdown through subsurface avalanche.

Non-isolated drain extended PMOS 450 and NMOS 470 can be modified intodevices wherein the drain is not surrounded by the gate on all sides.FIG. 14 shows schematic cross-sections of asymmetric extended-drain CMOSdevices including PMOS 500A and NMOS 500B, wherein drain extensions abuta gate on one side and field oxide on one or more other sides.

The asymmetric drifted PMOS 500A is formed in N-well 502 and includes P+drain 505B with an intervening P− drift region 507A of length L_(DP1)between the P+ drain and gate 511A on one side. A second P− drift region507B of length L_(DP2) is interposed between drain 505B and field oxidelayer 516. Drift region 507A, drift region 507B, and source extension506 may be formed using the same implantation step, such as PLDD2 of theprocess flow described above, or they may be separate implants that areindividually optimized for their specific function. The L_(DP2) andL_(DP1) of drift regions 507A and 507B may also be individuallyoptimized for their function. For example, the length and resistivity of507B are important for determining the BV of PMOS 500A, but do notaffect the on-state performance or hot-carrier reliability (HCI) of thedevice, while the doping and length of drift region 507A haveimplications for BV, on-resistance, and HCI.

Similarly, the asymmetric drifted NMOS 500B is formed in P-well 503 andincludes N+ drain 504B with an intervening N− drift region 509A oflength L_(DN1) between the N+ drain and its gate 511B on one side. Asecond N− drift region 509B of length L_(DN2) is interposed betweendrain 504B and field oxide layer 516. Drift region 509A, drift region509B, and source extension 508 may be formed using a common implantationstep, such as NLDD2 of the flow described above, or they may be formedby separate implant steps that are individually optimized for eachspecific function. The L_(DN2) and L_(DN1) of drift regions 509A and509B may also be individually optimized for their function. For example,the length and resistivity of drift region 509B is important fordetermining the BV of the NMOS, but do not affect the on-stateperformance or hot-carrier reliability (HCI) of the device, while thedoping and length of 509A have implications for BV, on-resistance, andHCI. In one embodiment, the BV of drift region 509B is intentionallymade lower than the BV of drift region 509A so that the breakdown alwaysoccurs far from the gate 511B.

In a preferred embodiment, source extension 508 is doped heavily toprovide low resistance from the source to the channel of the NMOS, whiledrift region 509A has a different doping profile that is optimized tosupport the drain breakdown voltage and provide good HCI. In anotherembodiment, drift region 509A also includes a second area of higherdoping near the N+ drain region, to provide graded drift region dopingfor better trade-off between on-resistance and HCI. Drift region 509Amay also be implanted at higher energy to provide a retrograde profilethat improves HCI by allowing most of the current to flow farther fromthe sensitive gate oxide-silicon interface.

The construction and fabrication of PMOS 500A and NMOS 500B areotherwise similar to the PMOS and NMOS devices of FIGS. 12 and 13. TheP+ implant forms source 505A and drain 505B in PMOS 500A while it formsthe P-well contact 505C in NMOS 500B. Conversely, the N+ implant formssource 504C and drain 504B in NMOS 500B while it forms the N-wellcontact 504A in PMOS 500A. The gate oxide 510A and 510B of PMOS 500A andNMOS 500B may be the same or may be individually optimized.

The PB layer 466 and NB layer 486 illustrated in FIGS. 12 and 13 mayalso be employed in the PMOS 500A and NMOS 500B. Alternatively a deeperimplant can be used to force breakdown under the drain and into the bulksilicon. In PMOS 500A, an optional N-type region 498 can be masked andimplanted into N-well 502 to locally increase the concentration andlower the breakdown of the junction formed between P+ drain 505B andN-well 502. Similarly, in NMOS 500B, P-type region 499 can be masked andimplanted into P-well 503 to locally increase the concentration andlower the breakdown of the junction formed between N+ drain 504B andP-well 503.

Lateral Trench DMOS

Compared to the aforementioned “planar” MOS and DMOS transistors havinga gate that sits atop the silicon surface and forms an MOS channel underthe gate along the silicon surface, the lateral trench gated DMOStransistor (LTDMOS) utilizes a trench gate to control channel currentvertically down the side of an etched trench, perpendicular, notparallel, to the wafer surface. Unlike a vertical trench DMOS, where thechannel current flows vertically through the entire substrate and outits backside, an LTDMOS redirects its vertical channel current laterallyinto its drain before the current flows back to a drain contact on thetopside surface of the wafer. The LTDMOS is much more three-dimensionalthan conventional planar MOS transistors. The trench gate structure,while more difficult to manufacture than planar gate devices, conferscertain advantages to a device's electrical properties.

Using a trench opening of 0.4 microns or less, the gate consumes lesssurface area than conventional MOS transistors, especially five voltdevices which require 0.5 to 0.6 micron gate lengths; twelve voltdevices which typically require 0.8 microns or more; and voltages twentyvolts and higher which require even longer gate lengths up to 4 microns.So space saving is a simple benefit of a trench gate.

Another benefit of an LTDMOS is its ability to form a fully self-alignedgate using a series of chained implants of differing energies and doseto create box-shaped and other non-Gaussian and/or non-monotonic dopantprofiles without the need for high temperature processing or longdiffusions. These unique dopant profiles can be tailored to help reducedepletion spreading into the channel, suppress short channel effects,inhibit punch-through channel leakage and breakdown, and limit thresholdvariability.

Compared to the conventionally constructed lateral DMOS, the verticalimplant of the LTDMOS described herein is simple and expedient, takingonly seconds to implant the entire DMOS body without the need for hightemperature diffusion. This method is sharply contrasted to the 12- to24-hour high-temperature diffusion needed by DMOS device 105 in FIG. 3C,or by the exotic tilt implant of lateral DMOS 110 in FIG. 3D, requiringprecise wafer rotation during implantation to avoid directional mismatchof the devices with gate orientation. And unlike the conventional DMOSdevice 120 of FIG. 3G, the LTDMOS of this invention is fully-selfaligned to the gate, making breakdown and impact ionization moreconsistent and reproducible.

Another benefit of the three-dimensional structure of the LTDMOS is theability to separate regions of high current density from those of highelectric field, thereby suppressing the impact ionization and unwanteddrift conductivity modulation effects of device 70 in FIG. 2B. Thedevice can also be engineered to subject the gate oxide to very lowelectric fields, e.g. where the gate has to support only a couple ofvolts even when the device is in avalanche breakdown. Low electricfields across the gate allow thinner gate oxides to be utilized in thedevice's construction, reducing gate voltage drive requirements andmaintaining low on-resistance even for high voltage devices.

Because the LTDMOS contains its body region within its drain, andcontains its source within its body, it is convenient to utilize asource body short uniformly throughout a device and without necessarilyshorting the body to the grounded substrate. The ability to provide“local” body contact reduces the source-body shunting resistance R_(SB)thereby effectively suppressing or even eliminating the snapbackphenomena plaguing device 60 in FIG. 2A.

Another advantage is that by using all low-temperature processing,LTDMOS fabrication does not affect or otherwise influence theintegration of other bipolar and CMOS devices in the integrated process,and supports the inclusion and exclusion of devices and correspondingprocess steps in a modular fashion. With all low-temperature processing,fabrication is not limited to small diameter wafers.

FIGS. 15A through 15C illustrate some key steps in the alllow-temperature fabrication of LTDMOS devices according to thisinvention. Referring to FIG. 15A, the fabrication of LTDMOS 550 startswith the etching, oxidation, polysilicon deposition, and planarizationsteps to create trench gate 552 with polysilicon gate 554 and gate oxide553 in substrate 551. Deep drift (ND) region 555 is then introduced byhigh energy or chain implantation, to a depth typically close to thedepth of the bottom of trench 552 although shallower or deeper depthsare possible too. Forming the ND region deeper than trench 552 may, forexample, be used to further reduce the gate electric field in lateraltrench DMOS devices used in high-side switch applications.

In shown in FIG. 15B, the process further includes the formation ofas-implanted P-type body 559, preferably using a chain implant ofvarying boron implant energies and doses, selectively masked bypatterned photoresist 556. The body implant may precede or follow theformation of an optional N-well 557 which ideally comprises achain-implanted non-Gaussian N-well comprising at least a lower portionNW1B and an upper portion NW1, where in a preferred embodiment the lowerportion NW1B is more heavily doped than the upper portion, especially ifthe same N-well is to be included as a structural and electrical elementin other devices fabricated along with trench lateral DMOS 550. A secondN-well can be substituted for the first N-well as desired, for example,if the second N-well has a higher average doping than the first N-welldoes.

The presence of the P-type body 559 divides drift region 555 into tworegions, a region 555A pinched under the body 559, and a region 555Bthat is not pinched by the P-type body layer. As shown in FIG. 15C,implantation is used to form N+ source regions 560A and 560B and draincontact 560C. Another implantation is used to form P+ body contacts 561Aand 561B. Current I follows a vertical path down the side of the trench552 and a lateral conduction path first through pinched drift region555A and then expanding out into unpinched drift region 555B foreventual collection by N+ drain contact 560C. Optional N-well 557 mayhelp to reduce the on-resistance.

FIGS. 16A and 16B illustrate one possible structure of a lateral trenchDMOS 580 comprising a non-conformal, deep N-type drift region 582. FIG.16A illustrates in cross section a gate-centric design comprising trenchgate polysilicon 585 and gate oxide 584 surrounded by N+ source 587B, P+body contact 586B, chain implanted P-type body 583, ND region 582, N+drain contact 587A, substrate contact 586A, ILD 590, barrier metal 588,and interconnect metal 589.

Except for the bottom of the trench gate, the entire gate and drainstructure is contained vertically within and laterally enclosed byimplanted drift region 582 comprising a portion 582A not pinched by thep-body, and portion 582B, pinched by P-type body 583. The pinchedportion 582B extending from the edge of gate polysilicon 585 to the edgeof the P-body region 583 has a length L_(J) (denoting a JFET-likeregion), while the edge of the P-body 583 to the edge of the optionalN-well drain 591 is defined as a drift length L_(D1). Lengthening eitheror both of these drift region lengths L_(J) and L_(D1) increases BV ofLTDMOS 580 but also increases its on-resistance.

The outer termination of LTDMOS 580 between the N-well drain 591 and theP substrate 581 comprises an extension of ND region 582A for a lengthL_(D2) and a substrate region of length L_(D3) bounded by P+ substratecontact 586A. The outer termination length does not affect deviceconduction properties the way L_(D1) doping and length do. The entiredevice is formed in P substrate 581 without the need of epitaxy.

FIG. 16B illustrates a top view of LTDMOS 580 including P+ substratering 602A, enclosing ND region 601, optional N-well regions 604A and604B, N+ drain contacts 605A and 605B, P-body region 603 with N+ sourceregions 605C and 605D abutting P+ body contact region 602B with trench609 and trench poly 608 contacted by contact window 607 where thepolysilicon is sitting atop the silicon surface outside of trench 609.Source contact 606 is shown as a butting contact straddling both N+source region 605C and P+ body contact region 602B. Alternatively, thesource and body contacts may be separate.

Drift lengths L_(D1), L_(D2), and L_(D3) are identified with respect tothe defining dopant regions. A portion defined as a “unit cell”describes a portion which may be repeated to form larger devices as longas the ND region 601 and the P+ substrate ring 602A are also expanded toaccommodate the larger device. The N-well drain may also fully enclosethe body region 603.

FIGS. 17A and 17B illustrate several drain-centric variants of LTDMOSdevices, structurally similar to that of FIG. 16A except that the drainis surrounded by the trench instead of the converse.

FIG. 17A illustrates a drain-centric LTDMOS 620 comprising N-well deepdrain region 623 and N+ drain contact region 628E surrounded by P+ bodycontact regions 627B and 627C, chain-implanted P-type body 626A, 626B,626C, and 626D, trench gate polysilicon 625 and gate oxide 624, N+source regions 628A, 628B, 628C, and 628D, ILD 631, barrier metal 629,and interconnect metal 630. The entire device is formed withinnon-conformal ND region 622 and P-type substrate 621 without the needfor epitaxy.

Electrically, N-well 623 forms the drift region of the device, wheredrift length L_(D1) is defined from the edge of the P-body 626C to theedge of the N+ drain contact region 628E. Lengthening this drift regionmay increase breakdown somewhat but with the higher doping of the N-well623 may result in only minimal increases in breakdown despite a nearlylinear increase in transistor drain resistance. The substrate contactimplant and contact ring and the outer termination of the device are notshown but may be achieved by extending the drift region 622 beyond theouter body regions using a design similar to that of LTDMOS 580 shown inFIGS. 16A and 16B.

FIG. 17B illustrates an LTDMOS 640 with a conformal drift region. Thisdevice includes optional deep drain region 643, N+ drain contact region648E surrounded by field oxide layer 652, P+ body contact regions 647Aand 647B, chain-implanted P-type body 646A, 646B, 646C, and 646D, trenchgate polysilicon 645 and gate oxide 644, N+ source regions 648A, 648B,648C, and 648D, ILD 651, barrier metal 649, and interconnect metal 650.The entire device is formed within conformal ND regions 642A and 642Band P-type substrate 641 without the need for epitaxy.

Electrically, regions 642A and 642B form the drift region of the device,where drift length L_(D1) is defined from the edge of the P-body 646C tothe edge of the deep drain region 643. This may be approximately thesame as the length of field oxide layer 652, as shown, but this is notnecessary. The pinched drift region 642A extending from the gate edge tothe edge of the P-body region 646B has a length L_(J) (denoting aJFET-like region). Lengthening either or both of these drift regionlengths L_(J) and L_(D1) may increase the BV of the LTDMOS but will alsoincrease its on-resistance. The substrate contact implant and contactring and the device's outer termination of the device are not shown butmay be achieved by extending the ND region 642 beyond the outer bodyregions using a design similar to the device of FIG. 16.

FIG. 17C illustrates an LTDMOS 660 that includes a deep drain region 663and N+ drain contact 668C surrounded by drift region 662A, P+ bodycontact 667, chain-implanted P-type body 666A and 666B, trench gatepolysilicon 665 and gate oxide 664, N+ source regions 668A and 668B, ILD671, barrier metal 669, and interconnect metal 670. The entire device isformed within high-energy implanted drift region 662 and P-typesubstrate 661 without the need for epitaxy. As shown, the center of N+drain 668C is the center line of the symmetric device.

Electrically, ND region 662 forms the drift region of the device, wherethe drift length L_(D1) is defined from the edge of the optional N-well663 to the edge of the P-body 666B without the presence of field oxide.Unlike LTDMOS 640 of FIG. 17B, the drift region 662 in LTDMOS 660 is notimplanted through a field oxide layer, so the resulting depth of driftregion 662 is substantially constant along its entire length. IncreasingL_(D1) may increase the BV of LTDMOS 660, but it will also increase theon-resistance. The substrate contact implant and contact ring and theouter termination of the device are not shown but may be achieved byextending the drift region 662 beyond the outer body regions using adesign similar to that of LTDMOS 580 shown in FIGS. 16A and 16B.

FIG. 17D illustrates an LTDMOS 680 that includes an optional deep drainregion 683 and N+ drain contact 688C surrounded by conformal driftregion 682, comprising a pinched portion 682B, non-pinched portion 682Aunder field oxide layer 691, and non-pinched portion 682C under that isnot under field oxide layer 691. LTDMOS 680 also includes P+ bodycontact 687, chain-implanted P-type body 686A and 686B, trench gatepolysilicon 685 and gate oxide 684, N+ source regions 688A and 688B, ILD692, barrier metal 689, and interconnect metal 690. The entire device isformed within conformal implanted drift regions 682A, 682B and 682C andP-type substrate 681 without the need for epitaxy. As shown the centerof N+ drain 688C is the center line of the symmetric device.

Electrically, drift region 682A forms the drift region of the device,where the drift length L_(D1) is defined from the edge of the N+ draincontact 688C or optional deep drain 683 to the edge of the P-body 686B.The length of field oxide layer 691 may be the same as L_(D1), as shown,or these lengths may be substantially different. Increasing L_(D1) mayincrease breakdown but also will increase the on-resistance. Thesubstrate contact implant and contact ring and the device's outertermination of the device are not shown but may be achieved by extendingdrift region 682A beyond the outer body regions using a design similarto that of LTDMOS 580 shown in FIGS. 16A and 16B.

LTDMOS 700, shown in FIG. 17E, includes an N+ drain contact 708Esurrounded by N-well 703, P+ body contacts 707A and 707B,chain-implanted P-type body regions 706A, 706B, 706C, and 706D, trenchgate polysilicon 705 and gate oxide 704, N+ source regions 708A, 708B,708C, and 708D, ILD 711, barrier metal 709, and interconnect metal 710.The entire device is formed within high-energy implanted drift region702 and P-type substrate 701 without the need for epitaxy. As shown thecenter of N+ drain contact 708E is the center line of the symmetricdevice.

Electrically, N-well 703 and pinched portion of ND region 702 form thedrift region of the device, where drift length L_(D1) is defined fromthe edge of the N+ drain contact 708E to the edge of the P-body 706B.The pinched portion of ND region 702 extending from the edge of gatepolysilicon 705 to the edge of N-well 703 has a length L_(J).Lengthening either or both of these drift lengths L_(J) and L_(D1) mayincrease the BV of LTDMOS 700 but will also increase its on-resistance.Decreasing L_(D1) and/or L_(J) may cause N-well 703 to interfere withthe channel region.

The substrate contact implant and contact ring and the outer terminationof the device are not shown but may be achieved by extending the driftregion 702 beyond the outer body regions using a design similar to thatof LTDMOS 580 shown in FIGS. 16A and 16B.

FIG. 17F shows an LTDMOS 720 having an N+ drain contact 728E surroundedby drift region 722 comprising a portion 722A not pinched by P-body726B, and a portion 722B, pinched by P-type body 726B. The device alsoincludes P+ body contacts 727B and 727C, chain-implanted P-type bodyregions 726A, 726B, 726C, and 726D, trench gate polysilicon 725 and gateoxide 724, N+ source regions 728A, 728B, 728C, and 728D, ILD 731,barrier metal 729, and interconnect metal 730. Unlike LTDMOS 700 shownin FIG. 17E, LTDMOS 720 includes no N-well deep drain. The entire deviceis formed within P-type substrate 721 without the need for epitaxy. Asshown the center of N+ drain contact 728E is the center line of thesymmetric device.

Electrically, the drift region of the device comprises a first sectionwith length L_(D1) defined from the edge of the N+ 728E to the edge ofthe P-body 726B, and a second section L_(J) defined from the gate edgeto the edge of P-body 726B. Lengthening either or both of these driftregion lengths L_(J) and L_(D1) may increase the BV of the LTDMOS butwill also increase its on-resistance. The substrate contact implant andcontact ring and the outer termination of the device are not shown butmay be achieved by extended the ND region 722 beyond the outer bodyregions using a design similar to the device of FIG. 16.

FIGS. 18A-18C illustrate the structure of an LTDMOS 760 including a NDregion 762 formed conformal to field oxide layer 771, which ispreferably formed by a LOCOS process. As shown in the cross-sectionalview of FIG. 18A, LTDMOS 760 has a gate-centric design that includestrench gate polysilicon 765, gate oxide 764, N+ source 768B, P+ bodycontact 767A, chain implanted P-type body 766, optional N-well deepdrain region 763, N+ drain contact 768A, substrate contact 767B, ILD772, barrier metal 769, and interconnect metal 770.

Except for the bottom of the trench gate, the entire gate and drainstructure is contained vertically within and laterally enclosed byimplanted region ND region 762, comprising portions 762A, 762C and 762Dnot pinched by the P-body 766, and portion 762B, pinched by P-type body766. The pinched portion extending from the gate edge to the edge of theP-type body 766 has a length L_(J) (denoting a JFET-like region), whilethe edge of the P-body 766 to the edge of the N+ drain 768A or optionaldeep N-well drain 763 is defined as a drift length L_(D1). The ND region762 conforms to field oxide layer 771, thus being formed with ashallower depth and lower charge in regions 762C under field oxidelayers 771 and a greater depth in drain region 762A and in body and gateregion 762B. Lengthening L_(J) and L_(D1) increases the BV of LTDMOS 760but also increases its on-resistance.

The outer termination between the drain and the P substrate 761comprises an extension of drift region 762D for a length L_(D2) and asubstrate region of length L_(D3) bounded by P+ substrate contact 767B.The outer termination length and doping affect the BV of LTDMOS 760, butdo not affect its conduction properties. The entire device is formed insubstrate 761 without the need of epitaxy.

FIG. 18B illustrates a top view of LTDMOS 780 including P+ substratecontact 782, which is in the form of a ring enclosing ND region 781,N-well regions 783A and 783B, N+ drain contacts 785A and 785B, P-bodyregion 784 with N+ source regions 785B and 785C abutting P+ body contactregion 786, with trench 791 and trench polysilicon 790 contacted bycontact window 789 where the polysilicon is sitting atop the siliconsurface outside of trench 791. Source contact window 787 is showncontacting N+ source regions 785B and 785C since P+ body contact region786 is too narrow along the sides to be contacted. This design reducesthe width of P-body region 784 and the corresponding length L_(J) of thepinched portion 762B of the drift region. The P+ body contact region 786is contacted at the end of each finger by a separate contact 788.Alternatively the sources may be interrupted, periodically along thegate finger to facilitate additional P+ contact regions.

Drift lengths L_(D1), L_(D2), and L_(D3) are identified with respect tothe defining dopant regions. A portion defined as a “unit cell”describes a portion which may be repeated to form larger devices as longas the ND region 781 and the P+ substrate contact region 782 are alsoexpanded to accommodate the larger device. The ND region 781 may alsofully enclose the P-body 784.

In an alternative embodiment the width of the P+ and N+ regions alongthe gate may be alternated to support alternating contacts. Thisapproach is illustrated in FIG. 18C where source contact 808 contactsthe wider portion of N+ source region 805B, 805C and body contact 807contacts the wider portion of P+ body contact 806 in alternatingperiodicity. The entire structure is contained within the lateralfootprint of P-body 766. This design reduces the width of P-body 766 andthe corresponding length L_(J) of the pinched portion of the driftregion.

The remainder of this alternative embodiment includes P+ substratecontact 802, again in the form of a ring enclosing ND region 801,N-wells 803A and 803B, N+ drain contacts 805A and 805B, trench 811 andtrench polysilicon 810 contacted by contact window 809 where thepolysilicon is sitting atop the silicon surface outside of trench 811.

Isolated Lateral DMOS

Isolating an N-channel lateral DMOS without epitaxy requires the use ofa high-energy implanted deep N-type (DN) layer. The DN layer can beconsidered a replacement for a conventional epitaxial buried layer,which normally spans the interface between an epitaxial layer and anunderlying substrate, although the DN layer has unique properties thatdifferentiate it from its high-temperature predecessor, especially thatits formation doesn't require high temperature processing.

FIGS. 19A through 19C illustrate the use of the implanted DN layer inthe fabrication of an isolated lateral DMOS 840, starting with theformation of an implant contouring field oxide layer 844 preferablyusing a LOCOS process sequence. The DN implant is then selectivelymasked with photoresist 845 or other appropriate mask and the DNisolation layer 842 is formed by one or more high-energy implantationsof an N-type dopant, preferably phosphorous, into P-type substrate 841to form isolated pocket 843. The DN layer 842 has a shallower junctiondepth under field oxide layer 844, making a gradual transition from itsfull depth under active transistor areas to its depth under the LOCOSoxide. In a preferred embodiment the sidewall of the isolation pocket isself forming beneath the LOCOS bird's beak transition region.

As shown in FIG. 19B, a P-type well 847 is introduced into a definedactive area by ion implantation through photoresist mask 846. In thefinal device, this P-well will serve as the body of the lateral DMOS butbecause it precedes the gate formation, it is not self-aligned to thetransistor's gate. Unlike a conventional diffused well which has itspeak concentration near the surface and a monotonically decreasingconcentration with increasing depth, P-type well 247 is formed by highenergy ion implantation of P-type dopant, and preferably by a chainimplant comprising a series of boron implants varying in dose andenergy. The chain implant, while it may comprise any number of implants,is graphically represented in the drawing by two regions—a surface layerPW1, and a subsurface layer PW1B, formed by ion implantation through asingle mask and without the use of epitaxy. In a preferred embodimentthe doping concentration of the deeper layer is greater than that of thesurface well. P-well 847 may comprise the P-well used for integratingother NMOS transistors or it may comprise a dedicated implant. A secondP-well having different doping than the first P-well, e.g. PW2 and PW2B;may be substituted for the first P-well.

In FIG. 19C a deep N-type drift (ND) region 849 is selectively masked byphotoresist 848 and implanted at a high energy into regions withinisolated pocket 843. If implanted through LOCOS oxide 844, this NDregion 849 conforms to the field oxide profile, forming a “conformal”drift. Alternatively the ND region 849 may be formed in only activeareas. In another embodiment of this invention, the drift may comprise ashallow N− implant formed after the polysilicon gate, as described abovein connection with non-isolated extended-drain MOS devices. A gateoxide, polysilicon gate, N+ source, N+ drain, P+ implant for P-wellcontacting, and interconnection (not shown) are then added to completedevice fabrication. An additional N-well may also be used as a deepdrain or to complete sidewall isolation of the P-type pocket as needed.

FIG. 20 illustrates an isolated symmetric lateral DMOS 860 with aconformal deep ND region 864 that may be formed using the process flowdescribed above. The device as shown utilizes N-well 878 as a deep draincontact and as sidewall isolation overlapping a deeper portion 862B ofDN floor isolation layer 862 in active areas, not under field oxidelayer 873. The body of the isolated DMOS 860 comprises chain-implantedP-well 865 as described above, formed in isolated P-type pocket 863,sitting atop a deeper portion 862A of DN floor isolation layer 862 notunder field oxide layer 873. Conformal ND region 864 overlaps ashallower portion 862C of DN floor isolation layer 862, having ashallower junction depth in portions beneath field oxide layer 873.

DMOS 860 of FIG. 20 further comprises gate oxide 870, polysilicon gate871, gate silicide 872, P+ region 868A contacting the P-well and P+region 868B contacting the substrate, N+ source regions 867A and 867B,N+ drain region 867C, and lightly doped region 866 beneath sidewallspacer oxide 869. Metal 875 with barrier metal 874 contacts the devicethrough ILD 879.

Device 860 as shown is symmetric, with a line of symmetry at the centerof the P+ region 868A. Drift length L_(D1), the length of LOCOS oxide873, influences the breakdown of the isolated junction, i.e. the DMOSdrain to body breakdown, and also effects device on-resistance.Increasing the drift length to increase avalanche breakdown, however, islimited to a maximum voltage set by the breakdown between P-well 865 andportion 862A of DN layer 862. Drift length L_(D3), defined as the spacefrom N+ drain region 867C to P+ substrate contact 868B, determines thebreakdown of the isolated device to the surrounding substrate 861.

FIG. 21 illustrates an isolated lateral DMOS 880 without a field oxidelayer over the drift region. Chain-implanted N-well 883 forms the driftregion and serves as the sidewall isolation, overlapping onto DNisolation layer 882. Because the entire device is fabricated in activeareas without the presence of field oxide, the device does not utilizeany conformal junctions as in the previous example.

The body of the isolated DMOS 880 comprises chain-implanted P-well 884,formed as described above in isolated P-type pocket 885. The devicefurther comprises gate oxide 890, polysilicon gate 891, gate silicide892, P+ region 888A contacting the P-well 884 and P+ region 888Bcontacting the substrate 881, N+ source regions 887A and 887B, N+ drainregion 887C, and lightly doped N region 886 beneath sidewall spaceroxide 889. Metal 895 with barrier metal 894 contacts the device throughILD 893.

DMOS 880 as shown is symmetric, with a centerline at the center of theP+ region 888A. Drift length L_(D1), the space between N+ drain region887C and gate 891, influences the breakdown of the isolated junction,i.e. the DMOS drain to body breakdown, and also affects deviceon-resistance. Increasing the drift length to increase avalanchebreakdown, however, is limited to a maximum voltage set by the breakdownbetween P-well 884 and DN layer 882. Drift length L_(D3), defined as thespace from N-well 883 to P+ substrate contact 888B, determines thebreakdown of the isolated device to the surrounding substrate 881.

FIG. 22 illustrates an isolated lateral DMOS 900 using a shallow N-typeregion 909A and 909B to form the drift region. Sidewall isolation isformed using N-well 903A and 903B, overlapping onto DN layer 902.Because the entire device is fabricated in active areas without thepresence of field oxide, the device does not utilize any conformaljunctions generated by the presence of a discontinuous field oxide layerat the semiconductor surface.

The body of the isolated DMOS comprises chain-implanted P-well 904,formed as described above in isolated P-type pocket 905. The devicefurther comprises gate oxide 911, polysilicon gate 912, gate silicide913, P+ region 908A contacting the P-well 904, P+ region 908B contactingthe substrate 901, N+ source region 907A, N+ drain region 907C, andlightly doped N region 906 beneath sidewall spacer oxide 910. Metal 916with barrier metal 915 contacts the device through ILD 914.

Device 900 as shown is not symmetric, but instead includes agate-to-drain drift region of length L_(D1) and a P-well 904 to N-well903A space equal to the sum of L_(D2) and L_(D4). Drift length L_(D1),the space between N-well 903B and gate 911, influences the breakdown ofthe isolated junction, i.e. the DMOS drain to body breakdown, and alsoeffects device on-resistance. Increasing the drift length to increaseavalanche breakdown, however, is limited to a maximum voltage set by thebreakdown between P-well 904 and DN layer 902. Drift length L_(D2), thelength of N− drift region 909B, and L_(D4) the space between P-well 904and N− drift region 909B, affect only the breakdown but not thetransistor conduction properties. Drift length L_(D3), defined as lengthof N− drift region 909C, and L_(D5), the space from N− drift region 909Cto substrate contact 908B, determine the breakdown of the isolateddevice to the surrounding substrate 901.

JFET and Depletion-Mode MOS Devices

Another class of transistors that can be fabricated by the process ofthis invention are normally-on or depletion-mode field effecttransistors. Unlike enhancement-mode or normally-off transistors whichdo not conduct with their gate biased to the source (i.e. whenV_(GS)=0), normally-on transistors conduct drain current substantiallygreater than leakage current even for zero gate drive, i.e. I_(DSS)>>0.Depletion-mode devices are beneficial in startup circuits or toimplement constant current sources, especially for the high-voltageinput supplying bias current to switching power supply controlcircuitry. Once start-up is achieved and a switching regulator isself-powering, a normally-on transistor can be shut off to save powerand improve efficiency.

The normally-on transistors fabricated in this process architectureinclude N-channel depletion-mode MOS field effect transistors (orMOSFETs) and N-channel junction field effect transistors (or JFETs).N-channel normally-on devices exhibit a negative threshold (V_(TN)<0)and require an even greater negative gate-to-source bias to shut off orreduce the magnitude of drain current. Applying a positive gatepotential can within limits increases drain current.

The gate of a depletion-mode MOS transistor reduces channel current bydepleting the channel material of free carriers, using electrostaticcontrol to form a depletion region, hence the term “field effecttransistor”. Provided that the gate can fully deplete the channelregion, channel current of the device can be completely suppressed or“pinched-off”. If the depletion region is not deep enough, however, tocompletely deplete the channel, the device will always conduct somecurrent, a feature generally not desirable in power circuitapplications. In the steady state, the maximum depth of the depletionregion is limited by the formation of a surface inversion layer.Increasing the gate bias beyond this voltage does not increase the depthof the depletion region.

Because an MOS transistor has an insulated gate, its gate may be biasedto enhance or suppress drain current. For either positive or negativegate biases, the maximum safe gate voltage of an MOS depletion-modetransistor is limited to the gate oxide rupture voltage, derated forreliability purposes to around 4 MV/cm. Though the gate may be biased toeither polarity without conducting current, enhancing channel conductionby biasing the gate to accumulate rather than deplete channel carriersexhibits an asymptotic improvement in conductivity, and therefore is oflimited benefit.

In contrast to the insulated gate of a depletion-mode MOS transistor, aJFET utilizes a reversed biased P-N junction as a gate toelectrostatically induce a depletion region. Like the MOS-gated device,the reverse-biased gate to body (channel) depletes the channel ofcarriers to suppress drain current. Provided the depletion region canfully deplete the channel region, channel current of the device can becompletely suppressed or “pinched-off”. If the depletion region is notdeep enough, however, to completely deplete the channel, the device willalways conduct some current, a feature generally not desirable in powercircuit applications.

The maximum gate voltage to suppress drain current or shut off a JFET islimited by its drain-to-gate junction breakdown BV_(DG) orgate-to-source junction breakdown voltage or BV_(GS). In contrast, themaximum voltage to enhance conduction is limited to the forward biasingof the JFET gate, namely 0.6V for a silicon P-N junction gate. Enhancingconduction by biasing the gate to accumulate rather than deplete channelcarriers is asymptotic, and of limited benefit, especially consideringthe limited range in enhancing gate bias possible.

The integration of depletion-mode or JFET devices is not commonlypossible in conventional integrated circuit processes, especially foroperation at high voltage. Their fabrication often involves hightemperature processes and diffusion, offering poor control of the MOSthreshold or JFET pinch-off voltage. The devices of this invention,however, do not rely on high temperature processes and thereby offersuperior pinch-off control with low off-state leakage capability.

FIG. 23 illustrates a high voltage JFET 920 with ND region 922fabricated using the disclosed low-temperature process and preferablyformed using one or more high-energy implantation steps. In this device,N+ region 924A forms the JFET source, N+ region 924B and optional N-well923 forms the JFET drain, and P+ region 925B forms the JFET gate via thePN junction formed with the ND region 922. The portion of ND region 922that is pinched under P+ region 925B serves as the channel of the JFET,and the portion of ND region 922 extending from P+ region 925B to N+region 924B or optional N-well 923 forms a high voltage drift region oflength L_(D1). Some channel pinching occurs from the P-N junction formedbetween ND region 922 and P-type substrate 921, but this back-gateeffect is substantially less than the influence of the bias on P+ gateregion 925B. The pinch-off can be further adjusted by including anoptional P-body or P-base layer 926 as part of the JFET gate. The gate,source, and drain are contacted with interconnect metal 928 and barriermetal 927 extending through ILD 929.

The source voltage of JFET 920 may be floated to a potential above thesubstrate, e.g. as a high side device, by properly spacing P+ substratecontact region 925A from N+ source region 924A. This distance includes aportion of ND region 922 of length L_(D2) and a distance L_(D3) from NDregion 922 to the P+ substrate contact region 925A. As shown, the deviceis symmetric with the line of symmetry centered on drain N+ 924B.

FIG. 24 illustrates a depletion-mode NMOS 940 with a lightly doped drain(LDD) drift region 942A. Unlike a conventional enhancement mode NMOS orlateral DMOS, NMOS 940 has no P-well surrounding the source or otherwiseenclosing the device. The low threshold is set by the doping of lightlydoped substrate 941, the thickness of gate oxide 947, and gate material948. With proper adjustment of these parameters, a device thresholdvoltage of 0V to −1V is possible.

Depletion-mode NMOS 940 also includes N+ source 944A, N+ drain 944B,optional N-well deep drain 943, gate oxide 947 which may comprise afirst or second gate oxide, gate 948 with optional silicide 949,sidewall spacer 946, source extension 954, P+ substrate contact region945, field oxide layer 955, ILD 952, metal interconnect 951, and barriermetal 950.

Drift region 942, introduced subsequent to and self-aligned to gate 948and field oxide layer 955 surrounds and encloses drain 944B, laterallyextending to gate 948 as N-drift region 942A with a length of L_(D1),and laterally extending to the field oxide layer 955 as N− drift region942B of length L_(D2). To reduce the electric field at the edge of driftregion 942A abutting gate 948, metal field plate 953 may optionallyextend above and beyond the gate 948 and into the region above the driftregion 942A.

With a low concentration P-type substrate 941, preventing punch-throughbreakdown between drifted drain 942A and N+ source 944A requires a gatelength of polysilicon gate 948 exceeding minimum dimensions. Optionalhigh-energy implanted deep P-type (DP) layer 956 may also be used toprevent punch-through. This layer may overlap a portion of gate 948, asshown, or may extend more (overlapping a portion of 942A) or less (notextending beyond 954), depending on the doping levels and deviceconstruction. Depending on implant conditions, a tail of DP doping mayextend up to the channel area under gate 948 and influence the VT of thedepletion-mode device.

FIG. 25 illustrates a depletion-mode NMOS 960—with ND region 962implanted prior to gate formation. Unlike a conventional enhancementmode NMOS or lateral DMOS, NMOS 960 has no P-well surrounding the sourceor otherwise enclosing the device. The low threshold is set by thedoping of lightly doped substrate 961, the thickness of gate oxide 968,and gate material 969. With proper adjustment of these parameters, adevice threshold voltage of 0V to −1V is possible.

Depletion-mode NMOS 960 also includes N+ source region 964A, N+ drainregion 964B, optional deep drain N-well 963, gate oxide 968 which maycomprise a first or second gate oxide, gate 969, optional gate silicide970, sidewall spacer 967, N source extension 966, P+ substrate contact965, field oxide layer 970, ILD 971, metal interconnect 973, and barriermetal 972.

Deep implanted ND region 962 is introduced prior to and therefore notself-aligned to gate 969. Field oxide layer 970 surrounds and enclosesdrain 964B. ND region 962 laterally extends to gate 969 with a length ofL_(D1), and laterally extends to field oxide layer 970 with a lengthL_(D2). To reduce the electric field at the edge of drift region 962abutting gate 969, metal field plate 974 may extend above and beyond thegate 964 and into the region above ND region 962. With low concentrationP-type substrate 961, preventing punch-through breakdown between NDregion 962 and N+ source region 964A requires a gate length ofpolysilicon gate 969 equal to or exceeding a minimum dimension. A deep Player similar to DP layer 956, described above, could also be includedin NMOS 960.

FIG. 26 illustrates a depletion-mode NMOS 980 with a deep conformalN-type drift region 982 implanted prior to gate formation. The lowthreshold is set by the lightly doped substrate 981 and thin gate 989.As described above, DMOS 980 has no P-well surrounding the source orotherwise enclosing the device, so a device threshold of 0V to −1V ispossible.

DMOS 980 also includes N+ source region 984A, N+ drain region 984B,optional deep drain N-well 983, gate oxide 988 which may comprise afirst or second gate oxide, gate 989, optional gate silicide 990,sidewall spacer 987, N source extension 986, P+ substrate contact 985,field oxide layer 991, ILD 994, metal interconnect 993, and barriermetal 992.

Deep implanted conformal ND region 982, introduced prior to andtherefore not self-aligned to gate 989, surrounds and encloses drain984B, laterally extends to active gate 989 as drift region 982A with alength of L_(D1), corresponding to the length of field oxide layer 991.A portion 982D of ND region 982 laterally extends under field oxide onthe side not facing the gate with a length L_(D2). The depth of theconformal ND region 982 under field oxide layer 991, as shown byportions 982A and 982D of ND region 982, is shallower than the portions982B and 982C of ND region 982 that are located under the drain 984B andthe gate 989. With low concentration P-type substrate 981, preventingpunch-through breakdown between deep portion 982C of ND region 982 andN+ source region 984A requires a gate length of polysilicon gate 989equal to or exceeding a minimum dimension. A DP layer similar to DPlayer 956, described above, could also be included in NMOS 980.

FIGS. 27A-27C illustrate the three depletion-mode NMOS devices shown inFIGS. 24 through 26, modified to include a P-type subsurface shield.This shield is included to reduce the onset on NPN parasitic bipolarconduction and to suppress snapback effects.

As an example similar to NMOS 940 of FIG. 24, FIG. 27A illustrates adepletion-mode NMOS 1000 with a shallow N− lightly doped drain (LDD) anda subsurface shield 1002. The low threshold is set by the lightly dopedsubstrate 1001 and thin gate oxide 1007. Unlike a conventionalenhancement mode NMOS or lateral DMOS, NMOS 940 has no P-well extendingbeyond the source or into the channel under the gate, but it doesinclude chain-implanted P-well 1002A and 1002B extending from underLOCOS field oxide layer 1010 to beneath N+ source region 1015A.Depending on the doping concentration of P substrate 1001 and thethickness of gate oxide 1007, a device threshold of 0V to −1V willresult.

NMOS 940 also includes N+ drain 1015B, optional N-well deep drain 1003,gate oxide 1007 which may comprise a first or second gate oxide, gatepolysilicon 1008, gate silicide 1009, sidewall oxide 1006, a shortlightly-doped N source extension 1004 (an artifact of the sidewallspacer manufacturing process), shallow LDD drift region 1005, ILD 1011,metal interconnect 1014, and barrier metal 1013.

FIG. 27B illustrates a depletion-mode NMOS 1020 that is similar to theNMOS 960 shown in FIG. 25, with a deep N-type drift 1025 implanted priorto gate formation. The low threshold is set by the lightly-dopedsubstrate 1021 and thin gate oxide 1028 but with the addition of asubsurface shield 1022. Unlike a conventional enhancement mode NMOS orlateral DMOS, depletion-mode NMOS 1020 has no P-well extending beyondthe source or into the channel under the gate, but it does includechain-implanted P-well 1022A and 1022B extending from under field oxidelayer 1034 to beneath N+ source region 1023A. Depending on the dopingconcentration of P substrate 1021 and the thickness of gate oxide 1028,a device threshold of 0V to −1V will result.

NMOS 1020 also includes N+ drain 1023B, chain-implanted deep drainN-well 1024, gate oxide layer 1028, gate 1029, gate silicide 1030,sidewall spacer 1027, N source extension 1026 (an artifact of thesidewall spacer manufacturing process), deep implanted uniform ND region1025, field oxide layer 1034, ILD 1033, metal interconnect 1032, andbarrier metal 1031.

In another variant similar to NMOS 980 of FIG. 26, NMOS 1040 in FIG. 27Cillustrates a depletion-mode NMOS 1040 with a deep conformal ND regions1044A through 1044C implanted prior to gate formation. A subsurfaceshield 1042 comprises a P-well 1042A under LOCOS field oxide layer 1049,and a deeper portion 1042B extending laterally beneath N+ source region1045A. The low threshold is set by the lightly-doped P substrate 1041and thin gate oxide 1046. Depending on the doping concentration of Psubstrate 1041 and the thickness of gate oxide 1046, a device thresholdof 0V to −1V will result.

NMOS 1040 also includes N+ drain region 1045B, chain-implanted deepdrain N-well 1043, gate 1047, optional gate silicide 1048, sidewallspacer 1053, N source extension 1054, deep implanted conformal NDregions 1044A through 1044C, field oxide layer 1044, ILD 1050, metalinterconnect 1052, and barrier metal 1051.

As another embodiment of this invention, FIG. 28 illustrates a fullyisolated depletion-mode NMOS 1060 with a shallow LDD formed without hightemperature processing or diffusions. In this device, DN floor isolationlayer 1062 is overlapped by an annular sidewall isolation and deep draincomprising N-wells 1063A and 1063B with shallow ND region 1068Aself-aligned to gate 1071 and ND region 1068B self-aligned to LOCOSfield oxide layer 1076. The drains are contacted through N+ region1066B, metal 1074, and barrier metal 1073.

N+ source region 1066A abuts sidewall spacer 1069, while N sourceextension 1067 is self aligned to gate 1071 sitting atop gate oxide 1070and shunted by silicide 1072. P-well 1064, comprising a chain-implantedP-type region extends laterally beneath N+ source region 1066A tosuppress NPN parasitic conduction and snapback but does not extendsufficiently laterally to overlap beneath gate 1071 as it does in thecase of isolated enhancement-mode lateral DMOS. The threshold of theisolated device is set by the doping concentration of isolated pocket1065, which is preferably the same as the doping concentration of Psubstrate 1061.

P-N Diodes and Junction Terminations

Another important function in power circuitry is the need to clamp thevoltage on sensitive MOS circuitry to avoid the risk of accidentallydamaging thin gate oxides from noise spikes and limited duration voltagetransients. This may be done with the use of diodes, which may bereferenced to ground or may “float” in an isolated tub and have abreakdown voltage lower than whatever circuitry or component beingprotected. These voltage clamps are commonly called Zener diodes eventhough the actual breakdown conduction mechanism is an avalancheprocess, and not quantum mechanical tunneling. We herein use the termZener and voltage clamp interchangeably without regard to the junctionbreakdown's physical mechanism.

Because diodes available in conventional integrated circuit processesare formed using high temperature diffusions, the resulting high surfaceconcentration forces breakdown near the surface where avalanche carriermultiplication can damage sensitive oxide and adversely impact devicereliability or voltage stability. In contrast the diodes of thisinvention use as-implanted dopant profiles formed using high energy andchained implants without the need for high-temperature diffusions, andexhibit avalanche breakdown buried below the surface in the bulk siliconwhere damage is less likely.

FIGS. 29A-29C illustrate a variety of Zener diodes that can be made witha process according to this invention. For example, FIG. 29A illustratesN+ buried clamp diodes K1 and K2 with grounded anode connections. DiodeK1 uses a P-well as an anode; diode K2 uses a P-base or P-body region asthe anode. The cathode of diode K1 comprises an N+ region 1083self-aligned to a LOCOS field oxide layer 1087. The anode of the diodeK1 comprises a P-well 1084, which has a lateral dimension smaller thanand is laterally enclosed within N+ cathode 1083. P-well 1084 isconnected by P+ contact 1082 through an opening in ILD 1088 to electrode“A” formed of metal interconnect 1090 and barrier metal 1089.

Unlike a conventional diffused well which has its peak concentrationnear the surface and a monotonically decreasing concentration withincreasing depth, P-well 1084 is formed by high energy ion implantationof boron, and preferably by a boron chain implant comprising a series ofboron implants varying in dose and energy. The chain implant, while itmay comprise any number of implants, is graphically represented in thedrawing by two regions—a surface layer PW1, and a subsurface layer PW1B,formed by ion implantation through a single mask and without the use ofepitaxy. For example, P-well 1084 may comprise either the first orsecond P-well described in Table 1.

In diode K2, an implanted P-type anode 1087 is formed beneath andlaterally contained within N+ cathode 1087, having cathode connection1090 and anode connection A. The P-body type implant may comprise asingle high-energy boron implant or a chain implant. For example, P-well1087 may comprise either the P-body or P-base region described inTable 1. Typically, the main difference between P-body or P-base regionsand P-well regions is the specific doping profile, with the latterhaving a more heavily-doped subsurface layer than the former.

An isolated version of a buried Zener diode using a P-base or P-bodyregion as the anode is illustrated in FIG. 29B. Diode 1100 comprisesisolated P-type region 1103 containing and isolating diode 1100 from Psubstrate 1101. Floating P-type region 1103 is isolated by high-energyimplanted DN floor isolation layer 1102 and sidewall isolation N-wells1105A and 1105B having an annular shape and overlapping vertically ontoDN layer 1102. N+ cathode 1106 extends across the surface between LOCOSfield oxide regions 1108 and forms an electrical contact with theisolation structure of DN layer 1102, and N-wells 1105A and 1105Bthrough its contact with N-wells 1105A and 1105B. The N+ cathode region1106 is contacted through ILD 1109 and electrically connected by metal1111 with barrier metal 1110, labeled K. P-body or P-base anode 1104 iscontained within isolated P-type region 1103 and contacted by a P+region within isolated P-type region 1103. This P+ contact region istypically located in the third dimension extending into the page, so itis not shown. Contact to non-isolated P-type substrate 1101 isfacilitated by P+ regions 1107A and 1107B, which in a preferredembodiment form a ring circumscribing diode 1100.

An isolated version of the a buried Zener diode using a P-well region asthe anode is illustrated in cross section 1120 of FIG. 29C. Zener diode1120 is formed in an isolated P-type region 1131 which contains andisolates Zener diode 1120 from P-type substrate 1121. Floating P-typeregion 1131 is isolated by high-energy implanted DN floor isolationlayer 1122 and sidewall isolation N-well 1123A and 1124B having anannular shape and overlapping vertically onto DN layer 1122. N+ cathoderegion 1125 extends across the surface between LOCOS oxide regions 1129and forms electrical contact with the isolation structure of DN layer112 and N-wells 1123A and 1123B through its contact with N-wells 1123Aand 1123B. The N+ cathode region is contacted through ILD 1130 andelectrically connected by metal 1128 with barrier metal 1127. P-wellanode 1124 is contained within isolated P-type region 1131 and contactedby a P+ region within isolated P-type region 1131, typically in thethird dimension (not shown). Contact to non-isolated portion of Psubstrate 1121 is facilitated by P+ regions 1126A and 1126B, which in apreferred embodiment form a ring circumscribing diode 1120.

Unlike a conventional diffused well which has its peak concentrationnear the surface and a monotonically decreasing concentration withincreasing depth, P-well 1124 is formed by high energy ion implantationof boron, and preferably by a boron chain-implant comprising a series ofboron implants varying in dose and energy. The chain implant, while itmay comprise any number of implants, is graphically represented in thedrawing by two regions—a surface layer PW1, and a subsurface layer PW1B,formed by ion implantation through a single mask and without the use ofepitaxy. In a preferred embodiment the deeper layer is more highlyconcentrated than the surface well. Alternatively, P-well 1124 may havea different dopant profile to achieve a different breakdown voltage.

Another isolated buried Zener available in the disclosed process asshown in cross section 1140 of FIG. 29D comprises the parallelcombination of stripes of N-well to P-well junctions all contained in afloating island isolated from the substrate. The diode comprisesmultiple isolated P-wells 1144A and 144B contacted by P+ regions 1146Dand 1446C, and multiple N-wells 1143A, 1143B and 1143C contacted by N+regions 1145A, 1145B, and 1145C, all sitting atop high energy implantedDN floor isolation layer 1142. N-wells 1143A and 1143C form an annularstructure isolating the entire Zener from substrate 1141. The device iscircumscribed by LOCOS 1149 and P+ substrate ring 1146A and 1146B.Interconnection of the various stripes of Zener diodes is facilitatedthrough metal 1148 with barrier metal 1147.

Unlike a conventional diffused well which has its peak concentrationnear the surface and a monotonically decreasing concentration withincreasing depth, the first P-type wells 1144A and 1144B, along withfirst N-wells 1143A, 1143B and 1143C are formed by high energy ionimplantation, and preferably by a chain-implant comprising a series ofimplants varying in dose and energy. While the chain implants maycomprise any number of implant steps, they are graphically representedin the drawing by two regions—surface layers PW1 and NW1, and asubsurface layers PW1B and NW1B. In a preferred embodiment the deeperlayers NW1B and PW1B are more highly concentrated than the surface well,causing the breakdown of the Zener to occur at a location well below thesurface. Alternatively a second P-well and a second N-well having adifferent dopant profile can be substituted for either the first P-wellor the first N-well or both to achieve a different breakdown.

Cross section 1160 of FIG. 29E illustrates an isolated P+ to N-baseburied Zener comprising isolated P-type region 1163 containing andisolating said P+ to N-base buried Zener diode from P-type substrate1161. Floating P-type region 1163 is isolated by high-energy implantedDN floor isolation layer 1162 and sidewall isolation N-wells 1165A and1165B having an annular shape and overlapping vertically onto DN layer1162 and contacted by N+ regions 1168A and 1168B. P+ anode 1167A extendsacross the surface and forms electrical contact with the isolated P-typeregion 1163 and with chain implanted P-well 1164 containing N-base 1166.Breakdown is determined by the concentration of the buried interfacebetween P+ 1167A and N-base 1166. The P+ anode, labeled A, is contactedthrough ILD 1172 and electrically connected by metal 1170 with barriermetal 1169. N-base cathode 1166 is contacted by N+ in the thirddimension (not shown). Contact to non-isolated P-type substrate 1161 isfacilitated by P+ regions 1167C and 1167B, which in a preferredembodiment form a ring circumscribing said diode.

Unlike a conventional diffused well which has its peak concentrationnear the surface and a monotonically decreasing concentration withincreasing depth, first P-type well 1164 is formed by high energy ionimplantation of boron, and preferably by a boron chain-implantcomprising a series of boron implants varying in dose and energy. Thechain implant, while it may comprise any number of implants, isgraphically represented in the drawing by two regions—a surface layerPW1, and a subsurface layer PW1B, formed by ion implantation through asingle mask and without the use of epitaxy. In a preferred embodimentthe deeper layer is more highly concentrated than the surface well,causing the breakdown of the Zener to occur at a location well below thesurface. Alternatively a second P-well having a different dopant profilecan be substituted for a first P-well to achieve a different breakdown.

Another inventive P-N diode in the process is the termination used tofloat isolated P-type pockets to high-voltages above the substrate. Thepurpose of the termination edge is to shape the electric field at theedge of the N-type sidewall isolation, where typically the sidewallcomprises an N-well overlapping onto a high energy implanted DN floorisolation layer.

In the embodiment shown in FIG. 30A, the isolated P-type pocket 1204 isisolated by DN floor isolation layer 1202 and N-well 1203 and surroundedby P-type substrate 1201 and P+ substrate ring 1205A. In this example,the termination comprises metal field plates 1211 and 1212 extendinglaterally over ILD 1210. The termination has a length L_(D3) defined asthe distance from P+ substrate ring 1205A to N-well 1203.

In the embodiment shown in FIG. 30B, the isolated P-type pocket 1224 isisolated by DN floor isolation layer 1222 and N-well 1223 and surroundedby P-type substrate 1221 and P+ substrate ring 1225A. In this example,the termination comprises polysilicon field plates 1231 and 1232 atopLOCOS field oxide layer 1230, and metal field plates 1234 and 1235extending laterally over ILD 1233. In other embodiments, only thepolysilicon or the metal field plates may be used on either the P+ orN-well side of the termination. The lengths of the field plates andtheir spacing is preferably adjusted to increase the BV of thetermination. The termination has a length L_(D3) defined as the distancefrom P+ substrate ring 1225A to N-well 1223. In this embodiment N-well1223 extends laterally under LOCOS field oxide layer 1230, causing thebottom portion NW1B to approach the surface and form a junctionextension region that is shallower and more lightly doped than thecombined NW1 and NW1B, which may serve to reduce the electric fieldcrowding near the edge of DN layer 1222 and thus improve the BV of thistermination.

In the embodiment shown in FIG. 30C, the isolated P-type pocket 1244 isisolated by DN floor isolation layer 1242 and N-well 1243 and surroundedby P-type substrate 1241 and P+ substrate ring 1245A. In thisembodiment, the termination comprises metal field plates 1251 and 1252which extend laterally over ILD 1253 and LOCOS field oxide layer 1250.The termination has a length L_(D3) defined as the distance from P+substrate ring 1245A to N-well 1243. In this embodiment, N-well 1243does not extend under LOCOS field oxide layer 1250. Also shown is anoptional polysilicon field plate 1254 that overlaps the edge of N-well1243 and has a portion sitting over thin oxide 1255 and a portionextending over LOCOS field oxide layer 1250. In combination with metalfield plate 1152, polysilicon field plate 1254 allows formation of fieldplates with up to three different thicknesses of dielectric between thefield plate and the underlying silicon.

In the embodiment shown in FIG. 30D, the isolated P-type pocket 1264 isisolated by DN floor isolation layer 1262 and N-well 1263 and surroundedby P-type substrate 1261 and P+ substrate ring 1265A. In thisembodiment, the termination comprises deep ND region 1266 connected toN-well 1263 and extending under field oxide layer 1270 by a lengthL_(D3) and spaced apart from P+ substrate ring 1265A by distance L_(D4).The termination may also include metal field plates 1271 and 1272extending over ILD 1270.

In the embodiment shown in FIG. 30E, the isolated P-type pocket 1284 isisolated by DN floor isolation layer 1282 and N-well 1283 and surroundedby P-type substrate 1281 and P+ substrate ring 1285A. N-well 1283 mayextend under LOCOS field oxide layer 1290, as shown, to provide a firstjunction extension region formed by NW1B. In an alternative embodiment,N-well 1283 may surround N+ region 1287, as shown, but not extendlaterally under LOCOS field oxide layer 1290. In this embodiment, thetermination also comprises conformal deep ND region 1286 connected toN-well 1283 and extending under LOCOS field oxide layer 1290 by a lengthL_(D3) and spaced apart from P+ substrate ring 1285A by distance L_(D4).The termination may also include metal field plates 1291 and 1292overlapping onto ILD 1293.

In the embodiment of FIG. 30F, the isolated P-type pocket 1304 isisolated by DN floor isolation layer 1302 and N-well 1303 and surroundedby P-type substrate 1301 and P+ substrate ring 1305A. In thisembodiment, the termination comprises conformal deep ND region 1306including a portion 1306A in the active area with a length L_(D3A) and aportion 1306B under LOCOS field oxide layer 1310 with a length L_(D3B),and spaced apart from P+ substrate ring 1305A by distance L_(D4).

In the embodiment of FIG. 30G, the isolated P-type pocket 1324 isisolated by DN floor isolation layer 1322 and N-well 1323 and surroundedby P-type substrate 1321 and P+ substrate ring 1325A. In thisembodiment, the termination comprises shallow N− drift region 1326connected to N-well 1323 and extending under field ILD 1330 by a lengthL_(D3). P+ substrate ring 1325A and N− drift region 1326 areself-aligned to LOCOS field oxide layer 1331 and spaced apart by adistance L_(D4).

In the embodiment of FIG. 30H, the isolated P-type pocket 1344 isisolated by DN floor isolation layer 1342 and N-well 1343 and surroundedby P-type substrate 1341A and P+ substrate ring 1345A. In thisembodiment, the substrate 1341A comprises a region 1341B under LOCOSfield oxide layer 1350 and ILD 1351, having a length L_(D3) defined asthe distance from P+ substrate ring 1345A to N-well 1343. A portion ofDN floor isolation layer 1342 extends beyond N-well 1343 toward P+substrate ring 1345A to help reduce surface electric fields. Theextension of DN layer 1342 beyond N-well 1343 does not extend belowLOCOS 1350 in this example, so the depth of the DN layer 1342 issubstantially constant in the termination area.

In the embodiment of FIG. 30I, the isolated P-type pocket 1364 isisolated by DN floor isolation layer 1362 and N-well 1363 and surroundedby P-type substrate 1361A and P+ substrate ring 1365A. In thisembodiment, the substrate 1361A comprises a region 1361B under ILD 1372and LOCOS field oxide layer 1370, having a length L_(D3) defined as thedistance from P+ substrate ring 1365A to N-well 1363. A portion of DNlayer 1362 extends beyond N-well 1363 toward P+ substrate ring 1365A tohelp reduce surface electric fields. The extension of DN layer 1362extends under a portion of LOCOS field oxide layer 1370 in this example,so the depth of the DN layer 1362 is conformal to the LOCOS field oxidelayer 1370 in the termination area.

In the embodiment of FIG. 30J, the isolated P-type pocket 1384 isisolated by DN floor isolation layer 1382 and N-well 1383 and surroundedby P-type substrate 1381A and P+ substrate ring 1385A. In thisembodiment, the substrate 1381A comprises a region 1381B under LOCOSfield oxide layer 1390 and ILD 1391, having a length L_(D4) between theP+ substrate ring 1385A and the N− drift region 1386, and L_(D3) betweenLOCOS field oxide layer 1390 and N-well 1383. The P+ substrate ring1385A and ND region 1386 are self-aligned to LOCOS field oxide layer1390. A portion of DN layer 1382 extends beyond N-well 1383 toward P+substrate ring 1385A to help reduce surface electric fields. The DNlayer 1382 may be held back from LOCOS field oxide layer 1390 such thatthe depth of DN layer 1382 is substantially constant, as shown, or itmay alternatively extend under the LOCOS field oxide layer 1390 so thatit has a depth that conforms to the LOCOS field oxide layer 1390, as inthe embodiment of FIG. 30I. Shallow ND region 1386 is included as asurface termination extending from N-well 1383 to LOCOS 1390.

In the embodiment of FIG. 30K, the isolated P-type pocket 1404 isisolated by DN floor isolation layer 1402 and N-well 1403 and surroundedby P-type substrate 1401A and P+ substrate ring 1405A. In thisembodiment, the substrate 1401A comprises a region 1401B under ILD 1411.A portion of DN layer 1402 extends beyond N-well 1403 toward P+substrate ring 1405A to help reduce surface electric fields. Shallow P−drift region 1406 is also included as a surface termination extendingfrom P+ 1405A toward N-well 1403. The termination has a length L_(D4)between the P+ substrate ring 1405A and the edge of P− drift region1406, and a length L_(D3) between the edge of P− drift region 1406 andN-well 1403.

The various features shown in the termination examples of FIGS. 30A-30Kare illustrative of terminations that are compatible with the process ofthis invention and capable of optimizing the BV of isolated regions. Itis well within the scope of this invention to combine the features fromdifferent figures to arrive at the best termination structure for agiven implementation. For example, the multi-tiered polysilicon andmetal field plates of FIGS. 30B and 30C, the conformal DN layer of FIG.30I, and the N− drift region of FIG. 30I may all be combined, and manyother combinations of the disclosed elements are also possible. It isalso within the scope of this invention to modify the structures shownin accordance with known processing techniques. For example, it ispossible to add metal interconnect layers above the single metal layershown, and to use these layers as additional levels of field plates. Itis further possible to substitute the LOCOS field oxide by alternativefield dielectric schemes such as deposited and/or recessed field oxides.

While specific embodiments of this invention have been described, itshould be understood that these embodiments are illustrative only, andnot limiting. Many additional or alternative embodiments in accordancewith the broad principles of this invention will be apparent to those ofskill in the art.

1. A depletion mode MOSFET formed in a semiconductor substrate of afirst conductivity type with a background doping concentration, thesubstrate not comprising an epitaxial layer, the MOSFET comprising: afield oxide layer at a surface of the substrate, the field oxide layerhaving first and second openings formed therein, the first and secondopenings being separated by a segment of the field oxide layer; a sourceregion of a second conductivity type located adjacent the surface of thesubstrate in the first opening in the field oxide layer; a drain regionof the second conductivity type located adjacent the surface of thesubstrate in the second opening in the field oxide layer; a deep drainwell adjoining and being located directly below the drain region, thedeep drain well comprising at least two regions having different peakdoping concentrations, the at least two regions comprising an upperdopant region and a lower dopant region, the upper dopant region beinglocated directly below the drain region and directly above the lowerdopant region, the drain region having a peak doping concentrationgreater than a peak doping concentration of either the upper dopantregion or the lower dopant region, the lower dopant region having a peakdoping concentration greater than a peak doping concentration of theupper dopant region; a gate and gate dielectric layer overlying achannel region of the substrate, the channel region being locatedadjacent the surface of the substrate in the first opening in the fieldoxide layer; and a drift region of the second conductivity type, thedrift region abutting the deep drain well and extending from the deepdrain well laterally under the segment of the field oxide layer and intothe first opening in the field oxide layer, the drift region beingconformal with the segment of the field oxide layer such that the driftregion comprises a shallow portion and a deep portion, the shallowportion being located in an area directly below the segment of the fieldoxide layer and extending downward from the surface of the substrate toa shallow lower boundary, the deep portion being located in an areadirectly below the first opening in the field oxide layer and extendingdownward from the surface of the substrate to a deep lower boundary, thedeep lower boundary being at a greater depth in the substrate than theshallow lower boundary; wherein the doping concentration of the channelregion is the background doping concentration.
 2. The depletion modeMOSFET of claim 1 wherein the deep portion of the drift region abuts thechannel region.
 3. The depletion mode MOSFET of claim 2 wherein thefield oxide layer abuts the drain region.
 4. A depletion mode MOSFETformed in a semiconductor substrate of a first conductivity type with abackground doping concentration, the substrate not comprising anepitaxial layer, the MOSFET comprising: a field oxide layer at a surfaceof the substrate, the field oxide layer having first and second openingsformed therein, the first and second openings being separated by a firstsegment of the field oxide layer, the first opening being bounded by thefirst segment of the field oxide layer and a second segment of the fieldoxide layer; a source region of a second conductivity type locatedadjacent the surface of the substrate in the first opening in the fieldoxide layer; a drain region of the second conductivity type locatedadjacent the surface of the substrate in the second opening in the fieldoxide layer; a shield region of the first conductivity type locatedadjacent the source region, the shield region being conformal with thesecond segment of the field oxide layer such that the shield regioncomprises a shallow portion and a deep portion, the shallow portion ofthe shield region being located in an area directly below the secondsegment of the field oxide layer and extending downward from the surfaceof the substrate to a shallow lower boundary of the shield region, thedeep portion of the shield region being located in an area directlybelow the source region and extending downward from the source region toa deep lower boundary of the shield region, the deep lower boundary ofthe shield region being at a greater depth in the substrate than theshallow lower boundary of the shield region; wherein the shield regioncomprises an upper dopant region and a lower dopant region, the upperdopant region being located directly above at least a portion of thelower dopant region, the lower dopant region having a peak dopingconcentration greater than a peak doping concentration of the upperdopant region; a gate and gate dielectric layer overlying a channelregion of the substrate, the channel region being located adjacent thesurface of the substrate in the first opening in the field oxide layer;and a drift region of the second conductivity type, the drift regionextending under the first segment of the field oxide layer and into thefirst opening in the field oxide layer, the drift region being conformalwith the first segment of the field oxide layer such that the driftregion comprises a shallow portion and a deep portion, the shallowportion of the drift region being located directly below the firstsegment of the field oxide layer and extending downward from the surfaceof the substrate to a shallow lower boundary of the drift region, thedeep portion of the drift region being located in an area directly belowthe first opening in the field oxide layer and extending downward fromthe surface of the substrate to a deep lower boundary of the driftregion, the deep lower boundary of the drift region being at a greaterdepth in the substrate than the shallow lower boundary of the driftregion; wherein the doping concentration of the channel region is thebackground doping concentration.
 5. The depletion mode MOSFET of claim 4wherein the upper dopant region is located entirely in the deep portionof the shield region.
 6. The depletion mode MOSFET of claim 4 furthercomprising a deep drain well adjoining and being located directly belowthe drain region, the deep drain well comprising at least two regionshaving different peak doping concentrations, the at least two regionscomprising an upper dopant region and a lower dopant region, the upperdopant region being located directly below the drain region and directlyabove the lower dopant region, the drain region having a peak dopingconcentration greater than a peak doping concentration of either theupper dopant region or the lower dopant region, the lower dopant regionhaving a peak doping concentration greater than a peak dopingconcentration of the upper dopant region.